You get this with THWN and THWN-2 as well and maybe some others.
THWN and XHHW are 90 C dry 75C wet. The ones with -2 suffix are 90C wet or dry.
I don't know what the actual physical difference may be.
Majority of time you find this marking along with one or two other markings on the conductor. Not so certain about the XHHW but pretty certain a lot of THWN-2 is actually marked THHN/THWN/THWN-2, not sure why exactly the -2 means it is wet or dry and 90C for both wet and dry, so additional markings are kind of redundant.
The additional markings indicate that it is "backwards compatible", when someone is looking for the product by the name of a previous generation of the product family. THWN-2 is all-encompassing of THHN and THWN, and mostly every other designation starting with a T in Table 310.16. Same with the XHHW-2 wire, for the wire types with X and H in that table. The "-2" means dual rated, indicating that it is 90C rated in both wet and dry environments.
There are less common environments where THWN-2 wire is still rated for only 75C, such as environments that require the gasoline and oil resistant product standard. Maybe an omni 90C rating will apply, when the wire type can withstand those enviornments at 90C.
Another example of redundant product ratings is 1000/2000V on wire, where it would logically follow that 1000V is automatically included in the 2000V rating. It simply makes it so you can find the product by either specification. That's the case for wire, unlike breakers where the slash rating has a much more specific meaning.
