Every field is optional and only the fields you send are written — anything omitted keeps its stored value. Send an explicit null to clear a nullable field.
Two different sets of defaults exist, and which one you get depends on whether a rule was ever stored. Creating a rule fills the fields you omitted with this schema's stated defaults and they are now stored values, yours to change. A priority you never wrote a rule for has no stored values at all: reading it returns is_customised: false and the platform-wide defaults for that priority, which are not the same numbers — they are deliberately stricter on low and looser on critical. So "the rule says min_interval_minutes 0" and "nothing was configured and 5 applies" are different states that both come back as a number.
Every hour in this schema — the quiet window, the schedule grid, the day the daily cap counts — is read on the platform clock, Europe/Kyiv. A recipient carries no timezone of their own, so a quiet window of 22–07 is 22–07 in Kyiv for everyone in the account, whatever local time that is for the person being paged.
Being over a limit does not postpone an alert indefinitely, and for two of the four it does not postpone it at all. A quiet window and a min_interval_minutes gap name the moment they end, so the alert waits and is delivered then — provided that moment falls inside the hour a send is retried for. Beyond that hour, and for the daily cap (which ends at midnight, almost always further off than an hour), the alert ends cancelled: it carries blocked_reason and the time it would have become allowed, and nobody re-sends it. An hour outside schedule_grid names no moment to wait for at all, so that one does not wait even briefly — it ends failed. Read the two statuses as they are meant: cancelled is the platform declining to page on your instructions, failed is a delivery that did not happen.
Only the first page is subject to any of this. Escalation steps, the owner fallback and test alerts bypass all four checks by design — an escalation that went quiet during quiet hours would be an escalation chain running with nobody at the end of it. So a quiet window mutes the opening page and nothing that follows it.