This is all on the interactive broker website. In the MATLAB doc you will find this link: https://www.interactivebrokers.com/en/software/api/api.htm. You can then search for "IOrder" which shows the documentation on what you are looking for. There is then a field which states Action() and provides the list of supported values: BUY, SELL, SSHORT.
Since MATLAB is just allowing you to connect to IB, the actual fields allowed are all defined by IB and you must refer to their API documentation.
