You can read this old post of me for a bit of "history" related to that.
That flag was added in the early beta of 7.22 because MikroTik needed a way to express what the built-in lookup rules did in the routing rules. Because the built-in lookup sequence was presented as normal routing rules in early 7.22 beta. In the RC versions that idea of listing the built-in lookup rules by default was scrapped, and now we have /routing/settings/policy-rules. But you can still recreate the rules from the beta with the new actions and flags.