When you archive a member the system only saves their name and DOB. This is so that your club records stay intact.
When you anonymize a member's account, the system will replace their name with random strings. This is so that your invoices and accounts will stay in tact.
When you fully delete a member's account, everything pertaining to that member will be deleted (swim times, invoices etc).
Once you fully delete or anonymize a member, this can not be undone.