Since there is no standard, the systems typically have to convert data to communicate with another system. The general method is for the external device vendor to make the definition, and for the PMS to be programmed to the device manufacturer's specification. Something as seemingly simple as a room number can be represented in many different ways. Each system has set it's own specification and no industry body has attempted to set a standard which may be used globally ensuring that data will be consistently represented between systems. Not only are there no standards for the data elements, there are also no standards as to the data element types.
Some systems use short integers, some use long.
Some use dates as strings, some use dates as integers. The method used to avoid these conflicts is to convert all data to string values and create a fixed length record where each field starts at a specified character and ends at a specified location. In order to create a usable standard, the data elements and the types of data must be standardized. It is clear, however, that the serial interface cannot be the standard of the future. We have evolved into a world where networking is the only acceptable method for establishing connectivity between systems.