Form Validation on extended MY_Model

I know that everyone mentions form validation needs to be done on the "model" side.
However I am having trouble wrapping my head around best practice for the following scenario:

I have a MY_Model that has the basic CRUD setup.

Let's say I now have a "user" table where I extend the MY_Model

In "insert" if I want to "insert" through backend I may need to put a lot more items (more "required" items); however if someone "insert"s their details in the register page of the website the form validations may completely be different. I may have "required" fields in backend that are not "required" in registration.

How do I handle something like this dynamically?