Difference between units and sets

Hi - I'm new to train planning, and I'm hoping that someone on here can help me with one of the basics: The difference between units and sets. Is there a resource that anyone could kindly point me towards, or possible give a simple explanation (visual representations welcome!) - thanks in advance from Dan

I'm only guessing here, but I'm pretty sure a unit is just 'a train', while a set is a full one. For example, a Class 373 is made of two trains connected in the middle. The set is made of two units.

However, they might just mean the same thing.

We use Unit/Formation

A single 4-Car EMU (electric multiple unit)
2x 4-Cars = 8-Car formation

These terms are somtimes used interchangeably, but broadly speaking a "unit" refers to one or more rail vehicles coupled together that is given a collective identifying number in the TOPS classification system. Rail vehicles coupled together that do not have a TOPS classification (sets) may be identified by other codes. In particular, Loco Hauled Coaching Stock (LHCS) may be designated with a "set" for identification purposes - examples include the new TPE LHCS that is numbered TP01 onwards, or HST coaching stock that is numbered with prefixes such as GW or NL. In this way, we can describe a "short set" to be a set of rail vehicles that are not as long as the rest of the fleet.

The way I see it is that a unit is a group of vehicles, powered or not, that are semi-permanently coupled together and not generally separated in operation. It will have a specific class identity in the TOPS system and the vehicles will usually have to visit a depot to be separated if required for maintenance or repair, although it might be organised elsewhere in an emergency. Sets or formations can consist of single or multiple units, including LHCS and locos that normally remain together for the duration of a diagram. TOCs will have set numbers allocated to manage this, as Roast Veg explains.

Unit = a multiple unit with a 6-character unit number e.g. 142001, 156401 etc. These have a unit class e.g. 142, 156.
Set = Loco-hauled or HST formations (a 'set' of coaches) with 4-character set numbers e.g. EC65, BN12, TP01 etc. These don't have a class as they aren't units. The coaches have Mark numbers e.g. Mark 1 or Mark 4 coaches. HST sets are sometimes given a 'class' designation (e.g. GWR sliding door short HST sets are 255 and the slam door short sets are 257) but these aren't real unit classes and are probably more for convenience when diagramming stock.
Having said all that the terms 'unit' and 'set' are often used interchangeably and the above is only my understanding of things.

If you are new to train planning I would ask your work colleagues. Whilst you will get accurate answers from some very knowledgeable folks on here, you'll also get some wild guesses from those that think they know. It's difficult sometimes to know what's correct. Anyway in this part of the world:
Units: Shorthand for Electric/Diesel/Bi-Mode Multiple Unit (EMU, DMU, BMU)
Set: Shorthand for Locomotive hauled (or pushed) coaching stock. But just to confuse you'll hear people refer to an 8 car or 12 car set when talking about EMUs.
Good luck with the planning, its a fascinating career if you have the aptitude.

Many thanks to everyone for their replies. Dan.