One of those hand held jobs that spits out the labels on a tape. They have different tapes, different fonts... and the darn labels seem to last forever. I have some on my boat put there by some PO that are in the sun and still are very readable... but I have reorganized my deck fittings and the labels are wrong now, but very difficult to remove. It's easier to cover them up with a new label. But it isn't likely that you would changed your electrical panel.

I used those same sort of labels to mark all the valves on the boat. They actually have a wire marker flag label that is extra tough for marking wire runs.

This is not the Dynamo embossed type thing from the 70s ('60s?), but a clean printed label from something like a Brother label maker.