BK means Black Edition. BK version is build on GA-Z97X-UD5H around durability and reliability just like TUF series from Asus. This gives users a peace of mind that the components on the mobo have been checked for longer duration of life span.
Motherboard features are: -
Durable components.
Safe design.
10Gbps M.2 connector.
10Gbps SATA Express connector.
Killer E2200 series NIC.
Intel LAN.
Realtek ALC1150.