So early printer manufacturers made the (quite reasonable) assumption that early adoptors would be nerds, and consequently would be playing TTRPGs. If those early printers were out of paper, they'd show the error message "PC LOAD LETTER" to indicate that they need to be refilled to be able to print character sheets. Of course they couldn't write "PLAYER LOAD PAPER", that would be too long and immersion breaking, so they made the printers act like NPCs by addressing their owners as "PC", and by talking about letters instead of just paper.

And it's an awful way to convey this. LTR PAPER LOW or NO LTR PAPER is clearer and uses fewer characters. But no, the committee wants the first two characters to be the acronym of the part that has an issue! Even TRAY1 LOW ON LTR or TRAY 2 OUT OF A4 fit on the 16-character display.