This is semantic but probably importantA) Bitcoin (UTXOs) *cannot* be stored offlineB) Bitcoin (UTXOs) exists on The Bitcoin *Network* (requires network connection)C) Keys can be stored offline. Keys are not Bitcoin. Keys prove ownership of Bitcoin (UTXOs) For you to spend Bitcoin you need two things- Keys- The Bitcoin NetworkWithout a network your keys become useless. Whether or not they are \offline\ doesn't matter.