True Fit partners with several stores. You go on a store's web site (for example, macys.com), and you tell True Fit about some clothing you already have that fits well (brand, size, style). Based on that, it refers to its database to recommend the best size of the product you're considering now, and it tells you whether it thinks it will be "snug", "true to you", or "loose".
Basically it just formalizes the kind of knowledge that shoppers use about how brand A tends to run small, brand B runs large, brand C is aimed at young people so its shirts are more tailored, brand D is aimed at older people and their shirts are more "generous" in the waist, etc.
Their web site calls it the "Fashion Genome™", which is a "comprehensive, normalized fashion graph".