Usage is the number of emails actually sent from the 1st to the last day of the current calendar month (campaign emails and flow emails together). It resets automatically each month. The Active Plan card at the top of the Billing page shows Emails sent, Remaining, Used %, a progress bar and N total emails this month (your plan maximum).

Before a campaign is saved, Sprwt adds the campaign's recipient count to what you have already sent this month; if the total reaches your plan limit the campaign is blocked with an upgrade prompt. That check only counts email sends, not SMS – flow SMS steps are checked against a separate (currently unused) SMS allowance instead. Flows check the email limit one email at a time and silently stop queueing emails once the limit is reached. If your Sprwt invoice is overdue, sending is also blocked with Billing - Please make sure you paid your invoice!.