What it is
Seam allowance is the fabric between the stitching line and the raw edge. Patterns are cut with it included; construction pages state it per seam type.
Why factories care
Inconsistent or unstated seam allowances change finished dimensions and fit. If your measurements are finished-garment specs, say so — the factory builds allowance into the pattern, not your numbers.
Ready to source in the UK?
Post your project on Combinate and get quotes from manufacturers who speak your language.