For those who do not play “SD Gundam Capsule Fighter Online“, the top level of performance customization that your Unit can achieve is Over Custom EX (aka OC 7).
[Although just recently, the South Korean server implemented a new upgrading system- amongst several other major changes to the game for Generation 6- which discards Custom/Over Custom in favor of a simpler Level 1-12 system. The Level System will eventually trickle down to all other international servers, including the North America server that I play on, but supposedly that will not happen for a few more years since it’s so new in South Korea.]
Anyways, OC EX is the most notoriously difficult level to reach, and necessitates using the Unit in PvP battles a specified amount of times before you can attempt EX on it. (In the case of the video below, C-rank Units require a minimum 200 PvP battles.)
Additionally, you must have Experience (EXP) points filled at the time you attempt EX. The danger of attempting any OC level is the possibility the Unit may reject the attempt, draining it of all its EXP points which are obtained through use in battles. However, easily-obtained EXP Packs can be used to artificially level up a Unit.
For these occasions (which are completely unpredictable), you use Level Protectors which act as a stopgap preventing the Unit from falling all the way back to the lower Custom (1-4) points, and will insure only the current OC level fails (as opposed to draining it entirely and having to start over from scratch).
I am aware of videos on YouTube showing dozens of attempts over months of time to reach EX on just one Unit.
(NOTE: These are my customized personal colors and decals on this CU-rank Z’Gok; it is normally yellow, red, brown, and green when you get it fresh from the capsule machine.)
–CAUTION: video features explicit language!–