Continuously is the default: the rule runs all the time, following the Frequency you chose. Specific Date makes the rule run once, at an exact date and time you set, which is useful for scheduled actions. Custom lets you select days of the week and, for each day, a time window (start and end). The rule only runs within those windows, with checks every 30 minutes. A tip for Custom mode: if you set the start time equal to the end time, the rule runs once at that time on the selected day.