When a Domain is created, 3 of the Territory Cards used to pay for the Princess are placed in the Domain along with the Princess.
Action cards from your hand can be stored on a Territory equal to or less than the cost of the Territory card instead of discarding them. These are referred to as “Action cards kept in your Domain.”
When Succession cards are stored on a Domain, they are not attached to the Territory cards in the Domain but are placed near your Princess.
When Succession/Territory cards are place in a Domain, they count as both Territory and Succession cards and can store cards like the Territories used to create the Domain. For example, if Imperial Estate is placed in your Domain, you can store cards on it as well as benefit from its Succession Points.