A contact is a human being. The system's address book: someone you call, not something you invoice.
The mobile number is required, the rest is not
Beside the name, the mobile number is the only required field, because in practice it is what makes a person unique. Email, national ID and position are all optional.
If you do enter a national ID it is checked for ten digits, and for being a duplicate.
Linking to a customer is optional
You can attach a contact to a customer and record their position there. You do not have to: a contact linked to nobody is still a valid contact.
One customer can have several contacts — a buyer, an accountant, a technical lead — all pointing at that same customer record.
A contact is not a login
Linking a contact to a customer does not give them portal access. Portal sign-in is defined on the customer record itself.
So if a person needs to be both in the address book and able to sign in, that person also needs a customer record. The two are not the same thing.
Delete means trash
A deleted contact goes to the trash. Its link to a customer goes with it, so if you restore one, check its position and link again.