Most product planning is a list of things the team could build, sorted by some flavour of priority. The list is always longer than the team can ship. The shipping budget is the harder, more honest constraint: how many things the team can actually move from idea to live this quarter, including the work nobody puts on the plan.
What the budget includes
Not just engineering days. Design, review, testing, launch coordination, support enablement, the inevitable bug fixes from the last thing you shipped, the carry-over from things that took longer than expected. The honest shipping budget is two to three times smaller than the naive one.
How to use it
Start with the honest budget. Sort the list by value. Draw a line where the budget runs out. Everything above the line is committed; everything below it is explicitly not happening this quarter. The line is the discipline. Without it, the team commits to the top of the list and the bottom slips silently, every quarter.
What this prevents
Roadmap padding. Quiet slips. The end-of-quarter scramble where five things ship at half quality because all five were committed. A real shipping budget produces fewer things, finished better, on schedule.