Assignment rules used to fire in position order: the first enabled rule with any overlapping tag won, even if a rule further down matched the ticket far more closely. That made shared tags a trap. If every rule in your set filtered on the same region tag, any ticket carrying that region landed on rule number one and the tag that actually distinguished it was never consulted.
Rules are now chosen by best match. Among the rules whose priority, team, and tag filters pass, the one matching the most of the ticket's tags takes it. Ties go to the more specific rule (the one with fewer non-matching tags in its filter), and only then to position. A rule with no tag filter scores zero, so catch-all rules naturally become fallbacks instead of overriding everything placed below them.
Rule sets with no tag filters behave exactly as before, so nothing changes for you unless you rely on tags. If you do, the one visible difference is that a catch-all placed first no longer beats tag-matching rules placed after it. No reconfiguration is needed; the new selection applies to the periodic sweep, instant assignment on ticket creation, and thread assignment alike.