I'm not sure whether I'm right, but will share my thoughts.
When you create the Item, you have access to all types of interfaces. I think depending on the Item type selection (Agent, SNMPx, IPMI) GUI should filter available interfaces.
For the other Item types may show all interfaces - I do not know.

And in general, what influences the type of interface?

And one more thing - for the Item type of SNMPx, I think we can hide the "Select" button for Items key helper. Maybe for some others too.