Production week is based upon a forward forecast of what cars are going to be built based on a 12 week schedule, 4 weeks confirmed, 4 weeks tentative, 4 weeks raw material.
Cars are built in batches of specs vs orders, but the 4 week confirmed should be built as per schedule, so long as the manufacturer doesn’t hit an issue, such as batch quality failure, supplier not meeting schedule etc.
This can mean some xars being brought forward or back.
build week is when the car is confirmed to be built a certain week. However, of course, things can happen on the build to stop that confirmed build.