If you need documentation on HFST version 2, you find it here. The page is no longer updated and it is recommended to update to version 3.

Some changes from version 2 to 3:

HFST version 2	HFST version 3
Namespaces HFST and HWFST	Namespace hfst and an ImplementationType argument
KeyLayer and SymbolLayer	Functions take a String argument, string-to-number mapping is done internally and automatically
TransducerLayer, Rule Layer, CoreExtensionLayer	HfstTransducer, namespace rules, class HfstTransitionGraph
Functions delete their transducer arguments	Functions modify their calling object and do not change their arguments (unless otherwise said)
No unknown or identity symbols	Unknown and identity symbols are supported in all implementation formats