B Corp certification assesses the whole company rather than one product line: how people are treated, what is bought and from whom, how the business is governed, and what is done with what it makes.
It is re-assessed, which is the reason it is useful in a supplier audit — it is not a badge earned once.
What to ask a supplier for
The assessment documentation, the materials sourcing list, and the workshop details. Together they answer most of what a procurement audit asks about a uniform supplier.
Made to order does the rest of the work: nothing is produced before it is ordered, so there is very little to write off.