Sorry for the confusion - low volume would be less than 15,000 messages monthly, a max of 6 messages per minute per campaign to AT&T.
Standard volume (may be referred to as high volume in the portal) is more than 15,000 messages a month or more than 6 messages per minute per campaign to AT&T. Please note there is no penalty if you choose standard volume and send less messages (so if you may be growing or sending more, this route is recommended).
As a reminder, no automated or API driven messages may be sent using a Conversational campaign type.