Complex sentence: TIA/EIA-568 specifies that horizontal cables should be terminated using the T568A pin/pair assignments, or "optionally, per (T568B) if necessary to accommodate certain 8-pin cabling systems".
Split into simpler sentences: TIA/EIA-568 specifies that horizontal cables should be terminated with the T568A pinout. The standard also allows the T568B pinout, as an alternative "if necessary to accommodate certain 8-pin cabling systems".