Here is an explanation from IBM about how Maximo handles work orders, work order hierarchies, and work order operations [via IBM Support]

From the user’s perspective, very little has changed in the existing work order functionality between Maximo 4 vs 6.x and 7.x. Work Order functionality have however been enhanced in the following two ways:

An increase in the amount of data that can be stored in operational tasks.
Additional functionality at the operational task level (for example, schedulability, ability to change status, and ability to expand an operational task into subtasks).

Prior to Maximo 4, work orders and work order operations were a simple two-level hierarchy in which the work order record acted as the header or parent record, and operations were the individual tasks that needed to be performed.

In Maximo 4, hierarchies were introduced to work orders. Hierarchies allow for planning larger projects. In this model, work
order operations are the leaves on the branches of the hierarchical tree. Operations are managed on the Work Order Tracking
application’s Plans tab.

In Maximo 4, each operation receives a number, typically 10, 20, 30, etc. The data is stored in the WPOPERATION table. You create job plans in the Job Plans application, complete with numbered operations.
You either apply plans to a work order directly via the Job Plan field on the main tab of the Work Order Tracking application, or indirectly by generating a work order via the Preventive Maintenance application where the PM record uses a preset job plan.

If there are multiple work orders that are all part of a larger project, then parent-child relationships are created between the
work orders on the Work Order Tracking application’s WO Hierarchy tab. “Operations” are the bottom level of that hierarchy.

Reasons for the Change

A number of clients have specific requirements that are not met by the Maximo 4 design. Three of the primary requests are listed below:

Scheduling – Operations cannot easily be individually scheduled Work Manager, Scheduler, or MS Project. The work scheduling solution in Maximo is based on MS Project. In Maximo 4, to use operations in scheduling applications, you have to assign a Labor resource to each individual operation.

For example, if a work order has ten operations, and each of those is to be scheduled separately, the Labor subtab (of the Plans tab in Work Order Tracking) requires ten separate entries, even if the same Labor or Craft resource is to perform them all.

Status – In Maximo 4, it is not possible to individually assign status to the operational tasks. If the job plan or work plan is complex or has a multiple-day duration, each task might need to be given its own status.

Subtasks – When you need to expand a specific operational task on a work order, there is no way to create subtasks suboperations under that task in Maximo 4.

Many of our customers have requested enhancements to address these limitations of work order operations. The sum of these enhancements began to make the work order operations nearly identical to work orders themselves.

In Maximo 6.x and 7.x, the WO Hierarchy tab has been merged with the Plans and Actuals tabs in Work Order Tracking. This lets you manage the entire work order hierarchy on one screen, the Plans tab, which shows both child work orders and work order tasks.
Instead of “operations” on the Plans tab, there are “tasks.”

For users moving from previous versions of Maximo, the new concept of hierarchies and operational tasks poses no significant change in the way you create work orders. Because our customers wanted operational tasks to have all of the features that work orders offer, Maximo now allows all the functionality of a standard work order at the operational task level of the hierarchy. What were formerly operations are now operational tasks. Tasks now use the same data model as work orders. Work order tasks are autonumbered.

A new column has been added to the WORKORDER table to distinguish between regular work order records and operational tasks. The column name is ISTASK; the on-screen field is labeled Is Task?. All regular work orders entered into Maximo will be inserted with the Is Task? flag set to “N.” When tasks are entered into the Tasks table windows on the Plans and Actuals tabs, the tasks are inserted into the WORKORDER table with the Is Task? flag set to “Y.” The Is Task? field is available on most search screens so that you can choose to display work orders, tasks, or both together.

The following benefits result from the enhancements to operational tasks:

1. Each individual task has all of the scheduling functionality of a regular work order.
2. Each individual task has complete flexibility in terms of assignments, supervisors, crews, etc.
3. Each task can be assigned a status; either automatically inherited from the work order it belongs to (this is the default), or provided manually if a task requires its own status.
4. You can easily convert any task to a child work order with tasks of its own.
5. If an individual task is complex, you can manage it with all the same tools you use for a regular work order.
6. You can charge individual tasks separately to equipment, locations, and/or GL accounts.
7. The Is Task? field lets you query against work orders and their tasks separately, or against both at the same time.

Frequently Asked Questions
Q: We create a lot of work orders. Won’t the additional work order we create for each task greatly increase the total number of database records?
A: No, there is no significant change in the number of database records created. The only change is in which table the records are stored. Instead of saving operational task records in the WPOPERATION table, Maximo saves them in the WORKORDER table.

Q: Will this cause performance problems?
A: Testing performed at MRO Software indicates that there is no performance issue. The data model design incorporated many steps to ensure proper performance. In fact, some queries are even faster now that there is no table join required between the WORKORDER and WPOPERATION tables. Queries now only need to check the WORKORDER table. This change does not significantly increase the overall size of the database or the number of records that must be selected. All that has changed is where the data is stored.

Q: Will our users have to update the status on each operational task?
A: No. By default, the task’s status is automatically updated when its parent work order status is updated. A task can have its status managed independently from its parent, but this is optional.

Q: When users search for their work orders, how will they know the difference between the parent work orders and the tasks?
A: We have worked hard to ensure that the functionality that users are used to is preserved. By default, Maximo treats only those work orders with an ISTASK=N flag as regular work orders. All ISTASK=Y records are treated as operational tasks. A user has to take a specific step to query for an operational task when searching for work orders. When on the search screen in any of the Work Orders applications, result sets do not by default include operational tasks.

Q: Won’t this use many more work order autonumbers?
A: Work Order Class was incorporated into Maximo. Each Class of Work Order can have its own autonumber seed and prefix. Tasks are ACTIVITY class work orders and traditional work orders are now WORKORDER class records. This allows different numbering systems for the work orders and the tasks. Your work orders can now be created in sequential order without the appearance of numbers being skipped.