Door frames and linings
The part of the order people forget until the door arrives and there is nothing to hang it on.
Range being built
You are almost certainly buying a lining, not a frame
For an internal door in a stud or block wall, the timber the door hangs on is a lining — a plain three-sided box with no rebate cut into it. The stop, the thin strip the door closes against, is pinned on separately once the door is hung and you know exactly where it sits.
A frame is the heavier, rebated version used for external doors and fire doors, where the rebate is machined in rather than pinned on. If someone sells you a "door frame" for an ordinary internal door, they usually mean a lining.
Lining width has to match your wall
This is the measurement people get wrong. The lining has to match the full thickness of the finished wall — studwork plus plasterboard plus skim on both faces — not the stud depth. Measure an existing reveal at several points before ordering; older walls are rarely consistent.
Standard widths exist for common wall build-ups, but if your wall falls between them you either pack the lining or order it wider and scribe it back. [VERIFY: supplier lining widths] — the widths we can actually get depend on the supplier range.
Fire doors change the rules
A fire rating belongs to the whole assembly — leaf, frame, intumescent seals, hinges and any closer — not to the door on its own. Hanging a certified fire door in an ordinary lining does not give you a fire door, and the frame specification is part of what was tested.
If the door is doing a fire-safety job, the frame is not the place to economise or improvise.