If you lease an aircraft, then you are making a legal agreement with the lessor, and they of course will want to maintain their asset and always have financial security of the asset. It means that the aircraft leasing agreement will have conditions for the installation of loan or 3rd Party components.
This can be a challenge in certain circumstances such as a leased aircraft for example where the title engine needs to be overhauled and the aircraft needs to continue in operation. For this to occur it means the engine must be removed (the title engine that came with the lease) and another engine must be installed. This engine being installed may well be a loan engine or a pool engine for example, meaning it does not belong to the lessee and it might also not be owned by the lessor of the aircraft.