Alerts monitor six metrics: CPA (cost per acquisition, based on pixel purchases), CPC, CPM, CTR (click-through rate, in %), Daily Spend (US$) and ROAS (return on ad spend). They always evaluate the current day's data (today), which is why spend is called Daily Spend. On cards and in notifications, CTR appears as a percentage, ROAS appears in multiplier format (for example, 2.50x) and the other metrics appear in dollars. If you need to evaluate longer periods (the last 7 days, for example), use Automated Rules, which have a configurable Calculation Period.