A numeric sender ID is a phone number, dedicated or shared, used as the origin of an SMS; it is the only sender type accepted in the Americas and the only one recipients can reply to.
Numeric senders are numbers: a dedicated long number rented by you, a short code, or a shared number assigned by the gateway. They are mandatory where operators refuse names and useful anywhere a reply is expected. A shared number is free and one-way in practice; a dedicated number costs a rental and enables conversations.
The downside is recognition: recipients see a number they do not know, so the text must identify the sender immediately.
At SMSMeteor: where a destination replaces names, a numeric sender is assigned automatically at no extra cost; a two-way number can be added per destination when replies are needed.