When you customize forms for a list or library, those forms simply replace or customize the current forms, the permissions are not affected. However, creating your own custom form that has nothing to do with a list that is already deployed can be affected in many ways. If the forms is deployed as a content type, then it can be used in any library that contains that content type. These lists or libraries can have specific permissions assigned to them, and the form will inherit those permissions. You can also assign item level permissions, but this will still be at the item or form level, not at the list or library level, and usually those permissions are not assigned until the item is created. Until that time, the form will inherit the permissions assigned to the list or library.