[!], [m5]... What are these and why?

Every time I unrar a .rar file, which contains a ROM, I get a list of rom to unzip. One says "this rom[!]" and "or that rom[m5]". I don't know what to do, so I pick the [!] one. What dose that mean ???? Thank you for reading this post.

If you're just looking to play the game, just use the one with the [!] symbol.I can't remember any of the other symbols besides that [b1] indicates a bad dump and it shouldn't be used.

Thank you, I wish I knew more about these abbreviation