This question appears in training and assessment material and it is worth answering properly, because the confusion behind it causes real problems in buildings: money spent on features that are not required, and features that are required left off because they were assumed optional.
What a fire door does require, everywhere
A leaf and frame of a tested construction, installed as the certification requires with the correct fixings and gap tolerances. Intumescent seals. Hinges of the specified grade, number and fixing. A latch or other means of holding the door closed against the pressure of a fire. Compatible ironmongery. And, where the door must close by itself, a self closing device.
What is location dependent rather than universal
Smoke seals, which are required where the door has to restrict cold smoke and not on every fire door. Self closing, which is required on most fire doors and not on all: a cupboard or riser door that is kept locked shut achieves the same outcome by being locked. Vision panels, which are a traffic and safety feature and not a fire requirement. Signage, which is a management requirement and varies with what the door is for.
What is not a requirement at all
A particular thickness. A particular weight. A particular colour or finish. A certificate fixed to the face of the door. An intumescent seal in both the leaf and the frame, when the certification calls for one or the other. A closer on a locked riser door. Each of these appears in specifications regularly and none of them is a rule.
The item most often wrongly omitted
Latching. A door with a closer and no latch will be pushed open by the pressure differential a fire creates, and a door standing open is no door at all. A closer without a latch is the most common genuine shortfall in an otherwise well specified doorset, and it is easy to miss because the door looks closed.
Questions people ask about which of these is not a fire door requirement
Is a self closer required on every fire door?
On most, and not on doors that are kept locked shut, such as risers and some cupboards. The requirement is that the door is closed when a fire occurs, and locking achieves that for a door nobody uses.
Are smoke seals always needed?
No. They are needed where smoke control is required, which includes doors onto protected escape routes and flat entrance doors. Elsewhere integrity alone may be what the guidance asks for.
Does a fire door have to be a specific thickness?
No. Thickness is an outcome of the construction that passed the test, not a requirement in itself, and different manufacturers achieve the same rating differently.