PS1 is not specific to bash. It is a standard environment variable meaning something like "prompt level 1". The syntax is even different for different shells. For example, in zsh, my PS1 looks like: "%n@%U%m%u:%~ %{^[[1m^[[31m%}%#%{^[[00m%}" which puts me in the "creative group" as I have a bright red end-of-prompt thingie.