The Application Of The Heaton Mint Mark

I would like to know the reason as to when or why the Heaton mint would apply their mark, as some coins produced by them have none.

Was it left up to the authority that commissioned the coin that decided to add the mint mark or was it the mint itself? Was it added only if Heaton engraved the dies themselves? Or perhaps only if there were two or more mints producing the same coinage and there happened to be a problem, it could be traced back to the mint that produced it?