what happens when i disable EDI_DC40 in mapping

i have a file to IDOC scenario,

in which there is a mapping between file and the IDOC structure,

can u guide me if i disable the EDI_DC40 segment of IDOC in the mapping, how exactly, the IDOC gets the value of

1. sndprn(sender partner number)

2. sdnpor(sender port)

3. rcvprn(receiver partner number)

4. rcvpor(receiver port)

and can i map this value explictly in mapping from the file..or so