Rob
Ansys Employee

Registers can use any variable field or one of the pre-set shapes. You're finding a few cells and separating them from the bulk fluid region. The source can then be added to the smaller region. 

We (Ansys) can offer training in most topics, and further support for our commercial users via the Customer Portal. UDFs aren't supported in the same way as the core code(s) but we can usually offer some guidance. We're more limited on the Community as it's considered "public" under the guidelines on knowledge export that we have to abide by. Training can only be delivered to existing customers, so you'd need to contact us (on ansys.com) from a "real" email and not Gmail/Yahoo/Demon etc.