Some of our libraries uses that number to notice that there was an error during a POST or GET request, if you are using one of them just follow the examples provided to know how to treat with that error constant.
If you are not using any of our libraries, I would advise you to check how your device handles errors during the requests creation and posting.
to return an error value or flag does not mean that errors are ‘common’, any routine firmware should always have a way to notice that something went wrong, i.e the Arduino official web client returns ‘-1’ if there is something wrong during a TCP packet reception or sending, and our libraries simply extends these kinds of alerts managing them in an global variable equals to the most negative int value (aka -340282336).
If you are using your project for business purposes, I strongly advise you to develop your own firmware to fit your project’s needs as our libraries are intended for being the starting point but not the final one, and as such, we need to generalize several stuff (like errors notifications) in order to extend the library’s functionality to most of the general use cases.