For example, someone like Hurricane Electric can't ask all of their customers worldwide to be in ARIN. Given the way this has historically developed, I don't think it's even reasonable to expect that only-RIR registries are used.
The provided list is the list of IRR (+ IRR combinations) that shows up in the pulldown menu in the admin interface of IXP Manager. It points to the where the AS-SET record is located for the directly attached member. So when you put the information in you select the IRR and type in (or pull) the AS-SET record. bgpq3/4 will then pull all the prefixes and generate a filter list from the union of all the prefixes listed. The AS_set name in the DB::NAME format will just need to be reformatted when entered into IXP Manager web interface.