It is sometimes possible to hear saying about one or another type of the bicycle "this a marketing term and means nothing", recently SUV being an example. Who decides if the new proposed category is "real" or not? Is it a matter of some existing authority or just a matter of consensus between cyclists?
I am asking about categories used my multiple sellers that do not assign them to obviously very different bicycles.
No one is "responsible" for it. It's basically community consensus.
If a company releases a new category of bike - an "SUV" bike, for example - and customers pick up on the name and start using it, other companies latch onto that name and start marketing their own "SUV" bikes. Eventually, enough companies are making that type of bike that we have a feel for what they have in common and what variances the community will accept and still consider it an "SUV" bike.
On the other hand, if nobody cares about "SUV" bikes, the category dies. (But probably gets "reinvented" under a new name a few years later.)
Eventually a company will come up with a new concept that falls outside of existing categories, give it a new name, and the whole thing starts over again.
To a large extent, bikes are categorized by their intended use. If a bike is labeled as a "road-racing bike," it probably meets UCI rules for road-racing bikes and is well-suited to that purpose. Likewise with other forms of competitive cycling: there's a clearly defined purpose and a governing body that decides what's "legal" for that type of event.
There's some fuzziness around the edges here. Gravel bikes became a new category 10 or so years ago, and quickly became popular. A lot of bike brands wanted to jump on that bandwagon. I've got a bike that was advertised as a "light gravel" bike but it's not really suitable for gravel (it's fine for my purposes).
Some categories come and go in popularity. In the 80s, a popular category was the "sport tourer," that is, not as sporty as a racing bike, not as relaxed as a touring bike. You'd be hard-pressed to find a bike advertised that way today. Similarly, road-racing bikes back then were categorized by the kind of race they were intended for, eg criterium bikes or 100-milers. They differed mostly in their geometry. These days, racing bikes are more likely to be classified as "aero" or "climbing" but have the same or very similar geometry.
I'm not aware of any organism in charge of the classification, except the UCI (Union Cycliste Internationale - International Cycling Organisation). That being said, the categories defined by UCI are pretty much limited to disciplines in competition, and may not match the colloquial definition (for example, there is now a "gravel" discipline, that is probably narrower than what the spectrum covered by gravel bikes today). But it still has a big impact on sporty bicycle designs, especially the ones that are linked to a UCI discipline: a cross-country MTB for example is expected to be compliant with UCI rules.
In areas not covered by UCI, there are no rules, and classification can be regional too.
There can be legal categories.
Here in Austria the road traffic regulations have certain exceptions for road bikes. A road bike is defined as a bike which weighs less than 12kg, has an outer rim diameter of more than 630mm, a rim width of less than 23mm and a “road bike handlebar”. There are also special rules for bicycles with more than one wheel side-by-side („mehrspurig“). We also have special laws for electric scooters.
For e-bikes, legal regulations apply. This Wired article outlines the differences between the 3 current classes in the United States:
Now, there are various categories of mountain, gravel, and road bikes. For instance, gravel bikes (at the time of writing) are splitting into all-road bikes that are like endurance road bikes with big tires, versus adventure bikes whose use case may overlap some with hardtail MTBs. MTBs have always had downhill and XC bikes, with some categories in between. Road bikes have generally split into performance versus endurance. The performance bike segment had generally been split into aero versus lightweight, but this situation may be evolving. Also, endurance bikes only really became a distinct category in mid to late 2000s.
The above categorizations are all driven by a combination of marketing, consumer demand, and consensus. I'm not aware of law or regulation that applies to this market sector. A lot of the splits are driven by demand by racers. Then, because bike companies effectively use racers to test and market bikes to consumers, those changes trickle down to consumers. However, bottom-up demand can also drive change; I think that endurance road bikes and gravel bikes in general are examples of this happening.
The ISO standard ISO 4210-2:2023 – City, Trekking, Mountain & Racing Bicycles in part Part I "Vocabulary" has section 3.1 "Bicycle type". I am not sure what is actually defined there as it is behind a paywall. Surely not SUV, but those types appearing in the title of the standard may quite reasonably be described there.
Such categories as trail, enduro, XC or downhill are mostly just a matter of informal consensus. There is no-one who would decide whether a particular bicycle fits such category or not.
It's really a matter of consensus, and that partly depends on usefulness.
It's useful to refer to "road bikes" or "mountain bikes", both of which categories can be subdivided, or "cargo bikes". Categories can overlap, as they can be based on purpose or design, so a "hybrid" or "city bike" (already overlapping categories) can serve as a "utility bike" or "shopping bike" (often a disparaging term, but definitely not always).
Even well-established categories are fuzzy round the edges. You might think it's pretty clear what a mountain bike is, but where exactly do you draw the line between an entry-level mountain bike and a hybrid with the same forks? Or is a "road bike" anything with drop bars? Or drop bars excluding cyclocross? (and gravel?) Or only a drop bar bike built for speed on paved surfaces, further excluding tourers?
Categorisation is hard in all areas - "is a hot dog a sandwich?" is guaranteed to start a debate.
