Latch is a redstone device block that Jeb planned to add.[1]
Use
What he was thinking of adding was a simple memory cell block. It would have one input (S) and one output (O) similar to the repeater block, but where O only would be set to S if the controller signal (C) is on. In other words, when C=1 then O=S, but when C=0 then O keeps its old value regardless of S.
The placement would be similar to the repeater, with S leading in and O leading out, but also with C coming from either left or right side.
