Okay cool, I think I get it. As far as selection goes, the more options the better. The use case I have in mind is for transactions at Bitcoin meetups. A large single transaction could be built coordinating many of the transactions taking place at the meetup simultaneously. For example, I sell maple syrup from one individual and buy coffee from another. This could be one payjoin transaction? Could the participants in the transaction choose to consolidate utxos as well? How does signing work? Would the users be looking at the same payjoin transaction on each of their wallets and hit send at the same time?