I know that there is a explanation here: https://frappe.github.io/erpnext/user/manual/en/setting-up/articles/managing-perm-level.html
but it seems I can’t really understand the concept.
Can someone give a alternative explanation ?
The thing I am trying to do is in a Custom DocType a specific field must be shown only to a specific DocType Role.
First of all this is not related to v7.
Second if you are looking to deny someone to read a specific field that is where the field level permission come into play.
So generally speaking all fields are LVL0 (Lowest) and anyone who has a READ right for a document can READ LVL0, now if you make perm level of a field as LVL1, then only those users who have read access to LVL1 would see (READ) that field.
I hope it clears the concept.