What do you want to do?

I want to register myself (and optionally my SO and an additional family member)

Most people will use this. You can register yourself, your spouse or partner and an additional family member in one step.

I've already registered but now I want to add my SO

This is for adding an SO after you've already registered.

I've already registered but now I want to add another family member

This is for adding a family member after you've already registered.