What's the difference between "discharge" & "quash" of 498a?

It is not the "discharge of petition" but "discharge of the accused".

If it is a criminal matter, then usually it is called "quashing of complaint" and not "quashing or petition".

When the criminal complaint against an accused is quashed, it also amounts to discharge of the accused. 

If there are more accused persons than one, or if the charge sheet has already been filed in the court, then a single accused can also apply for discharge. 

Discharge is at the stage when the arguement is about fixing charges and quashing is for the petition itself.

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