Understanding Proport_spread parameter value

For engine parameter PROPORT_SPREAD we have a value called receive 0 proportions/global proportions. Reviewing documentation it says as below. I am not clear with the explation and not able to clearly understand when zero would be used and when glob_prop would be used. Can some one explain this (if possible with an example)

receive 0 proportions/global proportions:

• For missing months that would have occurred after the first sale for this combination, assign 0 proportions.
• For months that could not occur in the range of first sale- end of sales, use glob_prop.
• In this case, for months that could not have been included, Demantra Spectrum checks the value of the proport_missing parameter and then does the following: - If proport_missing does not equal 1, then missing months receive the rolling average (glob_prop). - If proport_missing equals 1, then missing months receive glob_prop*delta.