Ideally, there'd be a feature within the Outlook calendar that shows a meeting as Accepted Flexible or Accepted Firm so that fellow team members can see what meetings can be moved around (e.g. internal meetings) vs. locked in (e.g. client meetings).
There's currently no way to coordinate a meeting when calendars look full without pinging each attendee to see what's flexible in their schedule. This feature would save a lot of time and effort in scheduling.
Marking a meeting as Tentative doesn't solve for this, as then it's not clear whether a user will attend the meeting or not. However, this could make sense to apply to Tentative meetings as well, e.g. Tentative Flexible or Tentative Firm.