HTTP POST is a request method supported by the HTTP protocol, commonly used on the World Wide Web. The POST request method is designed to request that the web server being communicated with open a connection channel allowing the transfer of data that is included in the body of the request.
The feature is linked with the internal HTTP webserver in the printer, which by default serves up a page with printer configuration information. The HTTP POST feature is controlled by the SetGetDo (SGD) commands, which are outlined below.
When printing is done via HTTP POST, the data sent to the printer must always be in ZPL® format, regardless of the printer’s device.language SetGetDo command setting.
No responses can be made via HTTP POST; it is not implemented as a bi-directional protocol. In practice, this means that no query commands (such as ^HH or ~HS) should be used via HTTP POST. Any data returned from the printer as a result of commands sent via HTTP POST will be discarded.
Click to read the full Application Note on HTTP Post