Calendar Sharing - global or individual?

We are running a shared instance of P6 in our Citrix Environment. For the most part, each project manager has their own scheduling environment that isn't accessible to other project managers. We discovered that with each project manager, they create calendars that are global and accessible to everybody. Permissions do not seem to be very intuitive meaning you either have or do not have access to calendars .. and if you do, you see all.