If you take a look at what a NES cart is, you'd see that PRG0, etc represents chunks containing the binary data of the PRG ROM. If there
are more than 1 PRG chips on the PRG bus, use PRG1, PRG2, PRG4, etc.
One PRG chip would have PRG0
Two PRG chips would have PRG1
etc.
Almost all only have one, so I'm guessing that PRG0 are images from newer carts where only one PRG chip was used and PRG1, etc. are from older carts where the density of the PRG (program) chips were less or that they used different memory management chips to allow more memory and PRG chips to be addressed. Or maybe PRG0 contains only the data from the first program chip and PRG1 contains data from the second. I have really no idea which, sorry.
As for "rev x"... Maybe an unknown revision?