Shift Register Data lines?
The gated serial (A and B) inputs permit complete control over
  incoming data; a low at either input inhibits entry of the new data
  and resets the first flip-flop to the low level at the next clock
  (CLK) pulse. A high-level input enables the other input, which then
  determines the state of the first flip-flop.


It's essentially an input enable. There's no point in defining one as Data In and the other as Enable since they can perform either function.

To answer your question, you can do a couple different things if you don't care about using an enable: Holding A or B high or tying A and B together will make the input always enabled.