Enrollment permission, should you choose to grant it, can be set up in two ways, and used as follows: 

Student-specific permissions are the default and recommended option for most courses. These are frequently used when students need to override a requisite, or course capacity. 

Generated permissions work best when instructor or department consent is required for all students seeking to enroll (such as 699, 990, and 999 courses). You must indicate either instructor or department consent on these class sections.

Other considerations:

• Unchecking the permission box does not disable permissions; it switches the course to generated permission.
• To avoid enrollment issues, do not change the permission type once enrollment has begun.
• Do not change from generated to student-specific permissions after even 1 permissions number has been granted.