What does the -SF suffix stand for?

I was reading through the latest deliveries on Planespotters.net, and PR-CND was delivered to Connect Linhas Aéreas in April. The type is a Boeing 737-400SF, which I know is a cargo variant. I’m just wondering what the -SF actually stands for.

My guess would be standard body freighter

Boeing: Freighters?

Or maybe it means special freighter

That exists for the 400 as well…

BCF stands for Boeing Converted Freighter (BCF)

Makes sense. Thanks for the insight!