There are 8 screen hole bytes in the bottom 8 text rows (64 bytes total) and 8 expansions slots, so the screen hole byte at offset "N" was often used to store up to 8 bytes of data[1] (one byte in each of the rows' screen hole area) by the expansion card's firmware in Slot "N". Overwriting those bytes could result in system crashes and hardware hangs.

[1] https://retrocomputing.stackexchange.com/a/2541/3653

Ah, I remember using the memory holes in the HIRES graphics memory for scratch-pad usage, but had forgotten about this part. I loved the Apple ][ since it (up to the //e) was capable of being fully understood by a single human being. Few if any computers since then have held that distinction