The treasury pallet has the following code:
/// The treasury's pallet id, used for deriving its sovereign account ID. #[pallet::constant] type PalletId: Get<PalletId>;
Some definitions of sovereign accounts are given here:
Are there two types of sovereign accounts in Substrate/Polkadot? One from a consensus perspective and the other from a general notion of what it means to be an account that is, for all intents and purposes, sovereign.