A user which has been set to 'Advanced' permission mode can receive far more finely defined access permissions than possible in simple mode.

A user in 'Simple' mode has one defined role which defines one singular set of permissions, which are granted to this user in every Channel the user has access to.

This might not be desirable in many circumstances. It's conceivable that you'd want to grant a user full control over the content & playlists in one channel, but want him only in a supervisory role in another, without the ability to change anything there.

To switch a user to Advanced mode simply hit the toggle while editing the user.

This activates the Advanced mode for this specific user. You'll see a notification that the user has existing simple mode permissions (stemming from his role and channels). You can migrate these permissions if you wish so.

The dropdown with the presets only serves one purpose: to give the user a basic permission set (analogous to the roles in simple mode). 

The 'User Groups' dropdown shows which user groups (if applicable) this user belongs to, and you'll be able to add or remove the user from various groups.

Custom permissions

Here you can define a set of custom permissions that should apply to only this user. This can be used both in addition or in lieu of user groups .

Show all current permissions

This enables you to view the consolidated permissions of this user. At the top you'll find the 'combined' view: it shows you the permissions granted to this user on a channel by channel basis. This is a consolidated view which takes into account presets and any user group this user is assigned to.

Below that you'll find the individual user groups this user belongs to, and which permissions and channel access is granted by those.

Note: if multiple sets of permissions (both via custom permissions and user groups) are affecting the same channel, the permissions are consolidated into one large set.

Example: Max is assigned to both Group A and Group B.
Group A and Group B both define permissions for an Example Channel.

If Group A, for instance, does not enable Max to upload files to the library, but Group B *does*, Max will be able to upload files to the library.

In laymans terms: each permission you add, either via custom permissions or a user group, is enabled.

You can switch a user back to Simple mode at any time.  Please note that any advanced permissions are removed and the selected user role is applied instead.