Data location specifies where a particular complex data type will be stored. Depending on the default or annotation specified, the location can be storage or memory. This is applicable to arrays and structs and can be specified using the storage or memory keywords.
As copying between memory and storage can be quite expensive, specifying a location can be helpful to control the gas expenditure at times. Calldata is another memory location that is used to store function arguments.
Parameters of external functions use calldata memory. By default, parameters of functions are stored in memory, whereas all other local variables make use of storage. State variables, on the other hand, are required to use storage.
