A RAMP test estimates the amount of watts a cyclist can sustain for 40-70 minutes (FTP).
So it’s basically an estimate of an estimate. 
You can certainly use RAMP tests to check FTP and verify your progress, but it’s only one facet of your overall fitness and capabilities.
Here are some Xert links that discuss this topic. You can find posts on other forums/blogs by searching the Internet for “FTP and RAMP tests”.
With Xert you don’t need periodic RAMP tests to estimate TP/FTP. Instead their system recalculates your fitness signature after every activity (or lack thereof over time).
I still do a RAMP test occasionally as a challenge but don’t rely on them for my fitness signature. As long as your weekly/monthly workouts include a variety of efforts (focus) your signature values should be valid.