As I understand it, anything with <define/> or <set/> in project.xml is automatically made available to templates in Lime/OpenFL projects. I guess that SET_ is added to the beginning of the name, to help avoid conflicts.
There’s a bit of documentation for that here: XML Format
Note that it will be converted to uppercase when you use it in the template. So if you use <set name="deploydomain" value="yourdomain.com" /> you must access it using ::SET_DEPLOYDOMAIN:: in the template.