Analog oscilloscope triggers
The 465B has two time base, A and B. The B time base is called delayed time base and can be used to zoom on a portion of a signal to see it in greater details. You can also trigger on this zoomed signal by using the trigger B (and the "DLY" is for "delayed" or "delay").

You can watch some videos from w2aew on youtube here. He has differents tutorials on Tektronix oscilloscopes.