This operation will be used for voting on all SMTs and allows users to vote with each SMT separately, each with different amounts of mana. This will allow you to upvote content with one SMT while down-voting it with a different SMT.

Great!

Comments can specify up to 5 SMTs that can vote on them, for a total of 6 assets (5 SMTs plus STEEM).

Do you envision this as using tags, or will the interfaces include another entry point to select up to 5 SMT?