Caller Identification, is a telephone service, that transmits a caller's number and/or name to the called party's telephone equipment during the ringing signal, or when the call is being set up but before the call is answered. Basically this allows you to know who is calling you.
By choosing a country, you are precising the national prefix.
If you chose Luxembourg for example, every outgoing call will be dialed with +352. If you want to make a call in France you have to dial it's national prefix first, then the number.
The codecs are the different protocols which choose how the voice is translated by your device. The device takes these protocols one by one until he finds the correspondent codec.
The DDI, also known as DID, stands for Direct Inward Dialing.
It allows you to assign a number (extension) for each employee or workstation without requiring a separate physical phone line, for each, to connect to the PBX. If you don't have a PBX but simply a SIP account this option should be disabled.