A 'doorset' is defined within BS EN 12519, describing a door product 'supplied complete with all essential parts from a single source and for which the main intended use is the access of pedestrians'.
We are generally referring here to 'Steel Doorsets' as they are the most common and popular.

A doorset is a complete and whole 'system' which will include (but not exclusively) door leaf, door sub frame, intumescents, smoke seals (if applicable), hinges, hardware, signage, glazing, door closer and so on. All of these designed and tested to perform as one unit by the manufacturer and certified as such with further certification possible by third parties, when there is a specific requirement with the door in terms of performance. It is particularly recommended for examples that a 'fire doorset' is factory prepared and delivered to arrive on site with all components from one source of supply and more importantly manufactured to a standard which is repeated the same every single time. it is quite mad to think a certified fire door can be produced on site by the assembly of a door panel and possibly all other parts from other sources and no attention to tolerances with certainly no testing post installation. How can this be fully compliant and proved to work effectively as intended?

All work on manufacturing a 'doorset' should be completed under factory production control and ideally audited by a third party to ensure the completed specification at the production point.
The doorset is manufactured to size, even if this is purpose made, and then produced within the test data remit of the manufacturer and model.

All this apart, there is a simple logic to purchasing a 'doorset'. The door and frame is already assembled and ready to go for installation therefore reducing masses of time on site. The majority of door referred to as 'doorsets' in the UK are steel but more and more other materials are being produced as doorsets now. Steel is simply one of the best materials in terms of strength, durability, security and safety and modern production methods now mean some steel doors are very hard to idenify as such, looking great and more ferequently now being used internally in offices and homes alike.