In Accounting you can organize any transactions by several dimensions: Sales Region, Activity, Project, Campaign. Other Dimensions are Business Partner and Product.
There are four entries for customized dimensions: UserElement1 UserElement2, UserList1, UserList2
I could not find any documentation for this. The best seems to be https://wiki.compiere.com/display/docs/Performance+Analysis, chapter "user defined elements".
I will give more information when I played with this... ;-)