Eventually, Hal gets to explaining that in a ModeX screen lump such as MXS or TIL, every fourth pixel is stored (1, 5, 9, 13, 17, etc, until the end of the picture), then every fourth + 1 (2, 6, 10,14, 18, etc, until the end of the picture), then every fourth + 2 (you get the idea), and finally every fourth + 3.
These groups of "every fourth" are called planes, for reasons which are more archaic than the format itself.
The rationale behind this odd storage is that the ModeX graphic mode used by the OHRRPGCE uses this format to display graphics on the screen. To this day, scientists still don't understand why, but it does none the less. This doesn't help OHRRPGCE Source Ports which don't use ModeX, but to this, Hal says:
"Source Ports?! Back in MY day, we had one operating system, DOS, and one programming language, Machine Language. That's right, we didn't even get real words, just 1's and 0's. You kids have it too good these days. Infact..."