Progress billing invoices a long job in stages rather than once at the end.
Why it exists
On a job running weeks, materials and wages go out from day one. Invoicing only at completion means the business funds the entire job — see working capital.
The schedule of values
The job broken into stages with a value against each, agreed before work starts. It is what each invoice is raised against, and what the customer checks it against.
Two things make it work. The stages have to be objective — a stage completed, an inspection passed — rather than a percentage somebody estimates. And the values have to be front-loaded enough to match when costs are actually incurred, since materials are usually bought early.
Substantial completion
The point at which the work is usable for its purpose, even if minor items remain. It commonly triggers the final billing stage, starts warranty periods, and shifts responsibility for the site.
Because it carries all of that, what counts as substantial completion belongs in the contract rather than being argued at the end.
The remaining items
A short list of outstanding minor work agreed at that point. Keeping it short and written is what stops the last stage being withheld indefinitely.
Retained amounts
A percentage is frequently held back until completion — see retainage. It is earned revenue that is not yet collectable, and it needs tracking separately or it is quietly forgotten.
Changes mid-job
A variation billed outside the schedule of values causes disputes. Agreeing it as a change order and adding it to the schedule keeps one document authoritative.
The point work passes to the customer, and everything that triggers, is completion and handover.