Select the Add General Mask ... option to display the Add General Mask dialog box, which allows you to specify the permissions you are associating with the general mask. If the menu item is dimmed, it means you already have this entry on your ACL. You can have only one such entry on an ACL.
When supported, a general mask specifies the maximum permission set effective for all ACL entries, except the owner's entry and the entry for the ACL's home cell. It is applied over the permissions granted to the requester by the ACL entries, and only permissions common to both are actually granted.
Select the permissions you want by clicking on the desired permissions displayed in the Permissions box. Note that if you do not select any permissions, no permissions are granted and no access is allowed.
For more information about masks, see ACL Mask Entries.