An outfit formula describes a shape rather than specific garments. Once you know that a mid-layer over a plain top with dark trousers works on you, any garment fitting each slot produces a working outfit.
Formulas are what make a capsule wardrobe fast to use in the morning. Rather than assembling from scratch, you fill three known slots.
Related: a personal uniform is an outfit formula worn nearly every day, with variation only in the specific pieces.