You're correct - you decide what to send and how your app interprets what it receives via the API.
Things to note are that LoRa messages are short (see the wiki) and your max airtime a day (and thus # of bytes) is limited. Also, the current way the things network is set up there's no ACK to your messages, nor can you send data back to a node. That means you send data and hope it arrives at the other end.
My 2ct.