The Invoices module turns tracked hours into money in two directions at once. Understanding the shape saves a lot of confusion later.
Two kinds of invoice
Billable: what a client owes for a project. One per project per pay cycle, priced at the project's client bill rate. Its lines are the members who worked on it.
Payable: what your organization owes a member for their hours. One per member per pay cycle, priced at their pay rate. Its lines are the projects they worked on.
They sit in two tabs of the same page. Same period, same hours, viewed from the two sides of the transaction.
The pay cycle
Your organization has one pay cycle, either weekly, fortnightly, monthly or annually, set in Settings → Finance. It decides the period an invoice covers and when it is cut. One cycle governs both kinds, so a client bill and a member payment always cover the same dates.
The four states
Draft: generated but not final. An invoice with any hours it could not price is held here, because a plausible-looking total that is quietly short is worse than one that stops and asks.
Issued: finalised, ready to send, and sent if you have emailed it.
Paid: settled. This is the state that locks things down.
Cancelled: voided. It stays for the record but is owed by nobody.
Marking one paid does more than change a label
Two things happen, and both are protections:
Those hours are now spoken for. A later invoice will not bill them again, because the paid invoice records exactly which days and projects it covered.
The rates behind it are frozen. You cannot change a rate in a way that would alter what has already been settled.
Numbering
Invoice numbers are issued in sequence and never reused. The same period generated twice does not produce two invoices for the same subject, because the second run recognises the first.
A payable invoice is not a payslip
If your organization also runs payroll, the two figures for the same period will differ, and both are right. An invoice prices tracked hours per project. Payroll pays a period, including leave and holidays, with tax and deductions applied. Both settle, and neither replaces the other.
Where to start
Set your pay cycle, then set your rates, then generate. In that order, because rates entered after hours are tracked mean re-pricing.