An alphanumeric is a custom sender id that is registered with Mobile Service Providers and that can be stamped on SMS messages being sent out to users. For example, if your business name is Wasiwasi, you can purchase that alphanumeric and brand messages going out from your account as being from Wasiwasi.
One important distinction between alphanumerics and short codes is that with alphanumerics, users will not be able to reply to that sender id. Also, note that alphanumerics are case sensitive.
Once you purchase an alphanumeric, you will have the option of using it as the sender id for messages originating from your account.
The sender id should have atmost eleven characters