There are helmets for everyone and every discipline. Coverage, weight, and venting are the primary factors most concern themselves with, though, if going fast on the road or track is a desire, aerodynamics can be considered as well.
There are national and international helmet standards. In the United States, all helmets sold have to pass the Consumer Products Safety Commission (CPSC) standards. It is probably the most stringent standard; helmets sold in Europe have to pass CEN (European Committee for Standardization) EN1078, which isn’t as strict. Full-face helmets are often certified both by CPSC and to the American Society for Testing and Materials (ASTM) F1952 standard, a voluntary standard. Once the standards are met, the helmet can be sold
Lately, MIPS, short for Multidimensional Impact Protection System, has been incorporated into many helmets. It is supposed to reduce rotational forces on the head from angled impacts. MIPS is a sliding layer that allows for 10-15mm of rotation on impact. Whether or not it makes a difference is subject to debate; POC, for example, briefly incorporated it into helmets, then later dropped the technology.
In many respects, the categories most helmets are put in are pretty unimportant. You can wear just about any helmet doing any discipline. Although a full-face helmet for your next road ride might be just a little much. But to each their own right?
The best mountain bike helmet is the one that suits your needs. If you prefer cross-country and trail type of riding, ventilation, and weight probably matter more, so a smaller, lighter helmet might work best. As your riding desires tilt more towards gravity riding, a helmet with more coverage that’s a bit larger and extends farther down around your noggin makes sense. The Fox Dropframe MTB helmet or the Troy Lee Designs A1 helmet could be the ticket for your enduro helmet needs.
Full Face Helmets should not be considered lightly. They are heavy and typically don’t vent well. If you’re doing any climbing, they can get uncomfortable fast. But, if going downhill is all you’re doing, a full-face helmet, like the POC Coron Air Spin MTB helmet offers complete head protection.
Road helmets can be used for road, track, or cyclocross and make the best gravel bike helmets--the more you spend, the lighter, smaller, and better vented the helmet gets. Aero road helmets have become a thing of late, so sometimes, it’s worth losing some venting for better aerodynamics. Since aero road helmets depend on air being forced through them to vent, they get warm when climbing. Long climbs, summer heat, or both tilt in favor of better-vented helmets. This is why many helmets offer both. Mavic’s Cosmic Ultimate Helmet is all about venting, while their Comete Ultimate MIPS splits the difference with a slippery shape and reduced venting.