Some functions from ERC-721 interface contain `payable` modifiers. 

`function transferFrom(address _from, address _to, uint256 _tokenId) external payable;`

I was wondering why it is `payable` and found from [EIP-721][1] that explains why. 

>Solidity issue #3412: The above interfaces include explicit mutability guarantees for each function. Mutability guarantees are, in order weak to strong: payable, implicit nonpayable, view, and pure. Your implementation MUST meet the mutability guarantee in this interface and you MAY meet a stronger guarantee. For example, a payable function in this interface may be implemented as nonpayble (no state mutability specified) in your contract. We expect a later Solidity release will allow your stricter contract to inherit from this interface, but a workaround for version 0.4.20 is that you can edit this interface to add stricter mutability before inheriting from your contract.

Does this mean that when a function in the interface includes `payable` and when it is implemented, it can be changed to anything? such as `public`, `view`,  `pure`?  or did it mean that it can only be changed to **nonpayable** which is `nothing`? I don't get why it is not

`function transferFrom(address _from, address _to, uint256 _tokenId) external;`

in the first place.

Also the definition `Mutability guarantees` is hard to understand. Can someone explain this in a simpler way please? 


  [1]: https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md