First and foremost, XP.
There are two types of experience you earn in the game, General XP and Mech XP. General XP is earned at a slow rate, and can be used to unlock the ability to buy and equip modules, as well as unlocking mech efficiencies. Mech XP is earned at a much faster rate, but can only be used for the specific mech variant you piloted while earning it, and only for efficiencies. (You can later use MC to convert MXP into GXP, but I don't recommend it unless you are rich.)
All actions, such as playing a match, killing an enemy, or even destroying components, produce both GXP and MXP, which can be used as soon as your mech is released from the match.
Now, Eliting and Mastering.
There are three "tiers" of bonuses that can be added to your mech, improving its abilities permanently, even if you sell it and repurchase it later. These bonuses stay even when you do not own the mech currently.
These tiers are "Basic", "Elite" and "Master". In order to progress from basic to elite, and from elite to master, you must have the whole tier above unlocked first for three variants of the chassis (Hero mechs count as a unique variant, and can be used to elite like any other). It is important to note that once you have the traits unlocked for one variant, you do not have to have it in your mechbay to make progress on other variants of the chassis.
In example. I decide to buy and elite a HBK-4G. (Example me doesn't care about the extra module slot, and ignores mastery) But I only have four mech bay slots. I only have two slots open, and the rest are occupied. I can buy the 4G and put it in one slot, and buy a HBK-4P for the remaining one. Once I unlock the basic efficiencies, I can just sell the 4P, and buy a 4SP.
Now, I unlock the basic efficiencies for the 4SP, and then sell it. If I want to, I can put something else in the last slot, lets say a Jenner, it doesn't matter. I can now unlock the basic efficiencies for the 4G (If I hav not already done so) and progress straight into the elite tier, even though two of the mechs I used to unlock it are not in my bay. If I later decide to master the 4G, all I have to do is buy the 4P and 4SP again. When I do, all I have to do is fill out their elite tier (And then, for all I care, sell them) and the mastery slot will be unlocked for my 4G!
It is important to at least unlock all the elite skills, even if you ignore mastery, because once they are unlocked, all the basic efficiencies are Doubled.
I hope that made things a little more clear for people wondering all the odds and ends of eliting. It is very simple, but still needs to be explained.
Edited by TheFlyingScotsman, 02 October 2013 - 06:37 PM.