If you forward calls to a SIP or IAX number you may include the DID number to be part of the dial string. When you use this special variable: ${EXTEN}, it will be replaced with the DID number being used at the moment.
Forwarding rule examples:
SIP/${EXTEN}@mypbxhost.net
IAX2/fromdc:2324tret@mypbxhost.net/${EXTEN}
This feature may be used when you have a lot of DID's and would like to use one redirect profile for all of them. Specifying number with ${EXTEN} allows you to distinguish calls coming through different DID's.