It is a safety cap on spending. The schedule never publishes more articles in a calendar month than this number (leave it blank for No cap). It also caps the subscription estimate, and behind the scenes Sprwt additionally never publishes more than what was billed for (articles already published when you saved plus the estimated remaining runs). When the cap is hit the tab shows Monthly article limit reached (X/Y). The schedule will automatically resume on the 1st of next month. and the This Month card shows X / Y articles and the amount charged. In the current version, if you save the field blank ('No cap'), the page can still display the 'limit reached' banner because the on-screen cap value is stored as 0 rather than truly unlimited — this is a display quirk only, and does not stop the schedule from actually publishing (the cron treats a 0 cap as unlimited, bounded only by what was billed for).