[TxMt] Which 'semantic class' to use for opening tm_proj?

Hi, I'm going to give a whack at transforming a .tm_proj file into 'Favorites' and .tm_project files upon opening them (or if they exist navigate there). Couple of questions:

This seems like an event thing. However, in the Avian bundle, all the similar commands define themselves as a callback in the semantic class field, rather than specifying an event. I'm not understanding the reasoning behind this, since it seems to blur the distinction between 'events' and 'classes'. Is there any way to determine whether we should be targeting an event or adding a class of 'callback'?

Which semantic class (or other options) do I need to use to target only opening tm_proj files by extension?

Thanks! Brandon