It’s RJ45 cable termination in alternate choice of configuration, but I’m not the techie to try and explain the benefits, warts, or differences between the two.
my VERY, VERY limited understanding:
It will say right on the cable and we always use A in Canada . Both will work, both are accepted standards. Some customers will specify one or the other. IT types tell me, you should use the same for the permanent wiring as the patch wiring to arguably avoid a little noise at the transition.
A Crossover cable is made by doing one end in T568A and the opposing end T568B. Most commonly in Canada it is wired with T568A but if you purchase pre-made patch cables (and from China) they will most likely be wired as T568B.