A visit is defined by an actual check-in! Milestone visits are the total check-ins across all locations in the MT tenant, all time!
To specify location milestones, you can use the location parameter within this condition!
Use cases:
Creating an autocampaign to reach out to clients when they hit milestones such as 100 visits!
Creating a segment of those who have less than 1 all time visits (ie have never been in the studio before) to target them with an offer!