HylaFAX(tm) is a sophisticated enterprise-strength fax package for class 1 and
2 fax modems on unix systems. It provides spooling services and numerous
supporting fax management tools. The fax clients may reside on machines
different from the server and client implementations exist for a number of
platforms including windows.

You need this package if you are going to install hylafax-client and/or hylafax
server.

Most users want mgetty-voice to be installed too.