In case DID is not provided for the call chan_capi will pass the call to Dial Plan extension "s".
If extensionis not present in the Dial Plan the the call is rejected. In opposite case Dial Plan receives the possibility to process the call. For example to receive extension using DTMF/MF or keypad information.
The extension "s" is not matched by templates (_X.) like other system extensions: "h", "i", "T", ...
Example:
[isdn-in]
exten => s,1,Answer
exten => s,n,capicommand(chat|test|m|1-2)
exten => s,n,Hangup
