This parameter lets you specify a text file that contains an eMail signature
A number of eMail clients can use a "Cut Mark".
This is a "special sequence that typically starts on a new line and consists of two dashes (minus signs) and a space followed by another new line".
which looks like this
This "cut mark" is recognized by many eMail programs and is used as a cut off point for signatures. That way, when someone replies to one of your messages, your signature (anything below the cut mark) is removed.
This FAQ for The Bat! eMail client has a discription of how it is used for one client.
You may need to use the full path c:\blat\file.txt.