I think it's some rules lawyer expert like people are saying. IIRC they brought it out because they were making a Reconciliation Budget Bill, which they do like only once or twice a year, and the rules are that bill has a lower threshold to pass (since everything is filibustered now you need 60% to basically do anything else) but it has to be budget related stuff. When there was a question on whether some progressive legislation counted, the parliamentarian came out to read the ancient scrolls and divine from the stars that it didn't count and couldn't be put in the bill.