What each Ethernet category actually does, and how far it does it.
Measure the whole path including patch leads at both ends, not the straight-line distance.
Cat 6: 1 Gbps to 100 m, 10 Gbps to 55 m
The sensible default for new runs. Ten-gigabit works over shorter distances. Two things matter more than the number printed on the jacket: the termination, since a poorly punched-down keystone will fail at gigabit on a cable rated for ten, and whether it is solid or stranded core, because solid copper belongs in the wall and stranded belongs in a patch lead that gets moved. Copper-clad aluminium sold cheaply as Cat 6 meets none of this and is the usual cause of a run that will not do PoE.
What this cannot tell you
Take this with you
Ethernet Cable Reference on RunTheTests — free browser-based sysadmin tools, no sign-up.