With a Number you can make external calls to PSTN and mobile using a Connection or Trunk.
Same Number can be shared for outbound calls by any number of Trunks at the same time.
Connections can't share a number. If a number is attached to a Connection it is inbound and outbound number of that Connection and can only be used by that Connection, however it still can be used for making outbound calls by any number of Trunks.
Selected Number appears as a caller number and displays as a Caller ID on a call.