A fixed-price project is sold for an agreed price, not by the hour — but the hours are still recorded, approved and read exactly as above, because the business still needs to know what the work cost. Billing such a project is progress billing, and it takes a company switch (§9.10) plus an agreed price on the project.
Mechanically it is the same billing run: the period's approved time is gathered and invoiced. The difference is the cap: across all billings, the total valued at the entries' billable amounts may never pass the agreed price. A billing that would cross it is refused, with the arithmetic shown — what this run comes to, what has already been billed, and what the price is — so the conversation happens before the invoice exists, not on the client's doorstep.
Two consequences worth internalising. The value already billed, for cap purposes, is counted the profitability way — regular hours at the rate, overtime at a flat 1.5 (§2.9) — and a project whose agreed price is fully billed simply has nothing left to bill: further approved time still records, still costs, and shows in the project's margin, which is precisely the honest picture of fixed-price work that ran long.