Hey,
There are a lot of threads here on FMA ranks, but the subject needs revisiting every once in a while. In the FMA originally there was no rank, much like in other Asian Martial Arts, and your skill was measured by the body of your knowledge and ability in combat. But as the 19th century gave way to the 20th and so on ranking became quite common in the FMA, just as it did in other Asian styles.
Currently FMA ranking depends on what system and/or organization you're learning or affiliated with. Some like Modern Arnis and Doce Pares, have belts. Some have different ranking systems that involve the ability to wear a certain color in class. For example, I believe that Derobio gives a Red Shirt as their highest rank. Other systems have rank only concerning instruction like the Inosanto Blend. Some simply have no ranking system at all. Others have a blend similar to Uechi Ryu which has only white and black belts. Ultimately it depends on your teacher and system. Some systems leave it totally up to the instructor whether they want to use ranking or not.
In the end there really is no standard ranking system that spans styles and organizations.