Question about the USER Context setting in FreePBX

I have been fooling around with trunking Asterisk/FreePBX and a ShoreTEL system. After a bit of tinkering I was able to make the SIP phones in the asterisk sides off-system extensions of the ShoreTEL system and get calls working both ways.

Getting calls from ShoreTEL->Asterisk to work caused the most issues. The “trick” seemed to be to set the USER Context to “Unavailable” in the incoming trunk settings. If anything other then “Unavailable” is in this box incoming calls do not work - Asterisk always responds to ShoreTELS invite with a 403/Forbidden.

I’m fairly new to all this and I stumbled upon this solution more by luck then anything else. Is there someone who can give be a bit of a rundown on what this setting does and why may need to be set like that?