Official specification of new alignment

Hi everyone. Is there an official specification of the new alignment structure in IFC4.3? We are moving our implementation from IFC4.1 to 4.3, and would like to make sure we understand correctly how it works. Additionally, do you know if the actual version (on the GitHub) is stable, or if modification of IfcAlignment are likely to happen before IFC4.3 FINAL is released ? Many thanks in advance for you help.