As Stuart notes, the zero indicates the number of times the pattern-action is to be repeated. It must be followed immediately by an action that refers to operand [1].
So, in your example, the pattern is *Z (scan for the next class Z operand (delimiter)). If one is found, it will be operand [1] and the action RETYPE [1] 0 will cause that particular operand to have its class changed to the null class (0). The leading 0 tells QualityStage to repeat for all occurrences.
Yes, you (they) could have used REPEAT instead (page 50 of
Pattern Action Reference manual). Most of us prefer the solution with the smaller number of keystrokes.
The leading number does not have to be zero. If it is a non-zero number, then QualityStage repeats that number of times. For example the following will retype just the first three delimiters.
This facility is not available with the REPEAT action.