Push feeds one calendar from another and never touches the source. Mirror goes both ways. How to choose, and what switching later deletes.
Pick Push if one calendar should feed another and never be touched itself. Pick Mirror if both calendars should know about each other.
You choose this in the sync composer, on the control between your two groups of calendars.
One direction. Events copy from the source to the target, and the source is never modified — nothing gets written back to it, ever.
Use Push when:
Both directions. Every calendar in the sync is both a source and a target, so an event created in either one shows up in the other.
Use Mirror when both calendars are yours and you want either one to be a complete picture. Two work accounts after a job change; a personal Google account and an iCloud account you use on your phone.
Mirror doesn't mean the two calendars become identical. It means each one gets a copy of the other's events, and the copies still obey your appearance settings. If your title setting is Title says "Busy", a mirror gives you real events on one side and blocks labelled "Busy" on the other, going both ways. That's working correctly, even though it doesn't look like a mirror in the everyday sense of the word.
An iCal or ICS feed subscription is read-only. Nothing can write to it, so a sync containing one can only ever be a Push, with the feed as the source. This isn't a plan limit and upgrading won't unlock it — it's how those feeds work. If you need two-way sync with iCloud, connect it as a CalDAV account rather than subscribing to its ICS link.
You can change direction on an existing sync. Going from Mirror to Push means the calendar that stops being a target no longer receives new copies, and the copies already sitting in it are removed. You'll get a count of what's about to be deleted before it happens; read that number before confirming, because it isn't undoable from inside the app.
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