Simplify the following text for a Grade 11 native English-speaking person.
Original: A unitary authority is a type of local authority that has a single tier and is responsible for all local government functions within its area or performs additional functions which elsewhere in the relevant country are usually performed by national government or a higher level of sub-national government .
Simplified: A unitary authority is a type of local authority that is responsible for all local government functions within its area .