Difference between res_fax and res_fax_digium

Digium’s Fax For Asterisk is a commercial facsimile (fax) termination and origination solution designed to enhance the capabilities of Open Source and commercial Asterisk, as well as Switchvox. 

Fax For Asterisk provides two components: res_fax and res_fax_digium.

res_fax

The res_fax Asterisk resource module adds fax termination and origination functionality in Asterisk. It provides the FAXOPT Asterisk dialplan function and the SendFAX and ReceiveFAX dialplan applications to enable the user to build highly-customizable fax solutions.

res_fax_digium

The res_fax_digium Asterisk resource module provides core fax processing functionality in the form of T.38 support and several supported fax modems – V.21, V.27ter, V.29, and V.17 – which achieve speeds up to 14,400 bps. 

More information about Fax for Asterisk, please read its manual,. you can download here