You can visit these items on display at Kennett Classic.
I am told these computers were most often used by Airline reservation systems, but this set of manuals originated from the Engineering Department of DuPont in Wilmington, Delaware.
Before the advent of the modern movable hard drive seek head, hard drives resembled spinning barrels or drums. While the drum spun, fixed-location heads on the inside of the drum would wait for the desired data location stored on the inside surface of the drum to come around to the head's location.
Drum storage was organized into "channels". To picture these channels imagine one etches equally the inside of the storage cylinder horizontally 12, 18, 36 or whatever number of times. These divisions were called channels and data was stored in the channels, magnetically.
Storage is thus optimized by limiting how many drum rotations are required to read the desired data stored there. A programmer might design their program so that an exact data address was not needed, desiring only that the data be placed in the same or nearby channel, limiting drum rotations to get it all. The CPU could sort and process later. Inefficient programs would cause the CPU to have to point each bit individually, waiting for the drum to spin to the desired memory location for each byte.