This was posted deep in another thread, but since it bit me on the rear during the early beta, here it is spelled out.

G64 is the path blend mode for LinuxCNC. Here's the manual page: G Codes

So, G64 P0.005 means to keep the cutter within 5 thou of the programmed path.

By default (and it's possible Tormach changed the default since they were considering it. I've not checked the latest betas) G64 - without P means to keep the best speed possible, no matter how far away from the programmed point you end up.

I've seen it round corners 1/8" or more when blasting through plastic in G64-without-P mode, and it becomes annoying when engraving. In the attached pic, the white lines are the programmed path and the purplish ones are the path the cutter actually took. These letters are about 3/16" tall.