Solidity has three data-location keywords in common use — memory, storage, and calldata. What does each mean?

storage = permanent, lives on-chain beyond the function; memory = temporary, exists only during the function and can be modified; calldata = temporary and read-only (cannot be reassigned).

State variables are automatically storage. Function parameters are temporary, so they must be memory or calldata. Use calldata when you will not modify the argument; use memory when you need to change it inside the function:

function addPerson(string memory _name, uint256 _num) public { ... }

You only specify a location for the special reference types — arrays, structs, and mappings. Value types like uint256 are handled automatically and reject a location keyword.

Go deeper:

• Solidity docs: data location — storage vs memory vs calldata in full, with assignment behaviour.