Bleed is a promise, not a margin
The extra millimetres are a contract with the guillotine, and 3 mm is not always enough.
What bleed buys
Bleed is the artwork that may be cut away. It exists so a trim that lands a millimetre inside the intended line still shows ink rather than white paper. Without it, a tight trim becomes a visible defect on every copy in the run.
Why the number changes
Three millimetres suits a sheet-fed job on a stable stock. Heavy cover stock moves more at the guillotine, and a laminated cover can shift again after lamination. A shop that quotes one bleed number for every job is quoting a habit, not a tolerance.
The check to keep
Before plating, open the PDF and confirm the page box equals the trim size and the bleed box extends on every edge that can be cut. Then look at the paper: if the stock is heavy or coated, raise the bleed and record why.