What is the difference between DESKTOP_SESSION, XDG_SESSION_DESKTOP, and XDG_CURRENT_DESKTOP?

I want to detect which desktop manager I am ruing, and I found out that there are three environment variables, DESKTOP_SESSION , XDG_SESSION_DESKTOP , and XDG_CURRENT_DESKTOP which could help me do so. But what exactly is the difference between these three variables? Thanks in advance.