Aurel, I don't know for sure.... but my guess is it is an ascii (not supporting wide strings) string pointer that is terminated by a zero, or Null. How it works I have no idea?
ASCIIZ is an ascii string (character size is one byte) that is terminated with $0.
"cmdline" would become a pointer to such a string.
(a pointer does not need to be "terminated" as the length is always depending on the OS - 32 bit on a 32 bit OS, 64 bit on a 64 bit OS)
A pointer is a location in memory that holds the starting location of something else (in this case a string).
So the pointer "cmdline" only knows where the string is but not what the string's content is...