The Portfolio Diversifier assigns them, and it solves your whole portfolio at once rather than filling contests one at a time.
It looks at every lineup in your pool and every contest in your entries file, then finds the combination that gives your full set of entries the best mix of upside and coverage. It weighs contests by entry fee and payout structure, so a $100 single-entry carries more weight than a $1 entry in a huge field.
Lineup groups are solved in order, with higher $/entry groups first by default, so those groups get first pick from the pool. Avoid Duplicates Across Groups is on by default and stops the same lineup from being used in more than one group.
You can change the order by dragging groups, and you can turn Avoid Duplicates off on small slates where you have more entries than good unique lineups.