Linked from
The 8 pages that link to Star formation efficiency, each with the reason it gives.
Super star clusterRelated: High efficiency helps a cluster remain bound as leftover gas disperses.
Young stellar objectRelated: It depends partly on how young stars disperse their natal gas.
Starburst regionRelated: It distinguishes rapid conversion of gas from a merely large gas supply.
Westerhout 40Related: Estimating it in Westerhout 40 tests how effectively its gas forms stars.