This can be used in conjunction with the machine type to see in the PE header to detect if the PE file is running on a compatible system. There are a few other useful memory-related variables including the size and virtual base of the code and data, as well as the application's version number (completely user specified, some update utilities use this to detect if a newer version is available), entry point, and how many directories there are (see below).
Part of the optional header is NT-specific. This include the subsystem (console, driver, or GUI application), how much stack and heap space to reserve, and the minimum required Operating System, subsystem and Windows version. You can use your own values for all of these depending on the needs of your OS.