What it means
See only free/busy lets another person tell when you are occupied, without ordinary event names or descriptions. See event details reveals the information in events covered by that calendar permission. More powerful roles can edit events or manage sharing. Google warns that broad public or organization-wide access can override a narrow named permission, so the effective audience matters more than one person's entry in a sharing list.
A real-world example
A manager receives named free/busy access, but the calendar was already made public with full details for a conference. The manager can still see event details because the wider public setting supplies that access to everyone.
What to do
Inspect general Access permissions and Share with specific people in the calendar settings on a computer. Remove stale broad shares, give the least privilege needed, and test from a recipient account. Review guest invitations separately because guests can see the event they were invited to.
The distinction that matters
Free/busy is not complete anonymity: a repeated block at a particular time can reveal patterns, and a directly invited guest may learn details even if their calendar share is limited. Full event details can include locations, video links and notes. A person with permission to make changes may have additional reach beyond viewing. Do not assume an event's Private setting fixes an already public calendar. If a workplace manages permissions, ask its administrator which rules are fixed and which can be changed locally.
Google Calendar permission meanings
