What is a dye lot?
A dye lot is a batch of fabric dyed together in a single run, and it is the unit at which colour is guaranteed to match.
Also called: lot, dyelot, colour lot.
Two pieces of the same style dyed on different days will differ slightly in shade, because dyeing depends on bath chemistry, temperature, timing and the base cloth itself — none of which repeat exactly. Mills accept this and ship it as passing.
The difference is usually small, often around one to two Delta-E units, which is invisible on a single roll and unmistakable when two lots are sewn into one garment and viewed together under strong light. That is why a dye lot is not a label but a constraint: fabric from different lots should not be cut into the same garment without the buyer approving it in writing.
In practice this means inventory has to be held per lot rather than per style. A distributor with 600 yards of a style split across three lots does not have 600 sellable yards for a single order — they have three separate pools, and the largest one is what can actually ship.