Linux ps -elf command output, what is Process Flag meaning?

Question

In output of ps -elf command in linux, we can recognize that process flag of 4 and 1, and process flag of 5, (sum of 1 and 4), but what is meaning of process flag 0 ?

unfortunately, man does not help us completely...

Answer 1

Flags are represented by the bitwise OR of their numeric value; this is called a mask. As long as the flags are incompatible, adding them gives the same result, but reasoning in terms of additions obscures the way to find flag values from the mask.

4 means “used super-user privileges”, and 1 means “forked but didn't exec”, as indicated in the manual. 5 means that both flags apply. 0 means that neither flag applies. 0 always means that none of the flags are set.