This event is called if you have font encoding set to automatic and the internal handler does not think it has good font in the stock-fonts to handle the given text.
ResolveFont(Note that this event also fires before internal decision is made on oriental languages such as Japanese, Chinese and Korean, so you can also use the event to substitute those languages with TrueType font if you want, even if built in PDF fonts can support those languages.
Substituting on those languages is not bad idea as some Linux PDF viewers do not have built in fonts to support those languages.