What's the difference between anniversary and calendar renewal?

Anniversary gives each member their own renewal date based on when they joined; Fixed date renews everyone on one common date, such as January 1 for a calendar year.

Members has two dated term models. You choose the default in Organization Settings → Terms & renewal rules under Default term.

Anniversary — the term runs from the day each member joins. A one-year member who joins on March 14 renews on March 14 the following year, so renewals arrive throughout the year.

Fixed date — everyone renews on one common date, set with Renewal month and Day. For a calendar year, choose January and 1. For a fiscal year, choose the day after your year end. The day can be 1 to 28, so it exists in every month.

With Fixed date, someone who joins part-way through the year is handled by the rule in When someone joins part-way through the year: prorate by month, prorate by day, no proration (they pay the full price for what is left of the year), or full price with the term running to the following renewal date after a cutoff month.

There is no separate "calendar" or "fiscal year" option. Both are a Fixed date. A level can also use its own term instead of the organization's default, including Rolling monthly and Life — no end date.

Only an Org admin can change these settings. Changes apply to terms created from then on; a term already running keeps its dates.