Hi Marcelo,
I suppose you can not assign a set type to categories belonging to two different hierarchies of same product type. In a standard CRM system set types like COMM_PR_SHTEXT, COMM_PR_MAT would be used in the category MAT_ of the hierarchy R3PRODSTYP. So, if you try to maintain the same set type for a category from different hierarchy, the list may not show the above mentioned set types.
'Send to ERP' would be required when you are trying to replicate the product from CRM to ERP for the first time. Any changes done for the product there after will be replicated to ERP automatically.
Hope it helps,
~Athreya