Learn English – Difference between “empathetic” and “empathic”

They’re not the same thing, though metonymy is often interpreted so widely that synecdoche can be regarded as a special case of it. You use synechdoche when you speak of a part of something but mean the whole thing. Example: "All hands on deck" is an example in which 'hands' is used to mean 'people'. You can also reverse the whole and the part, so using a word for something when you only mean part of it. Metonymy is similar, but uses something more generally or loosely associated with a concept to stand in for it. In other words, is the substitution of one word for another object or idea which it suggests or the substitution of the name of an attribute or feature for the name of the thing itself. Example: "The pen is mightier than the sword," meaning literary power is superior to military force. In the case of the sentence “Get away from me or I’ll hit you with the milk” the choice is difficult. That’s a case of metonymy all right, but you can argue it’s also synecdoche, because milk is an essential component part of a bottle of milk, not just something associated with it. I hope you'll find this explanation useful.

Always itself means all the time. So, both of them are very much interchangeable in usage.