In Maximo, you can use extra fields to store extra data and crossover fields is just an added functionality.  For example, in the asset table, there are extra fields such as eq1, eq2… and so forth.  These fields are there for you to use at your own free will and use them however you want.  Crossover fields is a functionality that allows you to pass data from one table to another.  When you create a crossover field, the data in that field is “crossed over” from that child table to a parent table.  You can use an extra field to cross over or you can use an existing field.  Let’s say you have set up a crossover field on SERIALNUM in the asset table.  When you create workorder against an asset, the SERIALNUM field will be crossed over to the workorder table and stored by the workorder object.  This is a very simple example but you can see how this functionality can be very useful.

To setup crossover functionality, read this article…