A permission is the ability to perform a particular action in Crucible, e.g. 'Create Review'.
Permissions in Crucible are managing through the use of permission schemes.
A permission scheme assigns particular permissions to any or all of the following: The scheme's permissions will apply to all reviews belonging to the project(s) with which the scheme is associated. You can create as many permission schemes as you wish. Each permission scheme can be associated with many projects or just one project, allowing you to tailor appropriate permissions for individual projects as required.