There's "Anion Exchange Capacity" as well.
In my opinion, it's far simpler to refer to it all as Exchange Capacity.
Each organic material / compound is comprised of different minerals, chemicals / nutrients and structure etc. These materials each break down at different rates, and release their ions at different rates. Water speeds up this process. It's these ions that are readily absorbed by the plant..
An analogy for myself is to think of a sponge.
We come along and poor water on it. The sponge reaches full capacity, and leaches some of what it was already holding. But also absorbes more water. An exchange happens.
It really is a very simplified explanation though.
Some materials don't absorb as well, or break down nearly as fast as others.
In gardening's case, it really is just a case of keeping a good balance of organic materials, and amendments.
One example for myself, would be each time i dress some top soil with green cow manure, i like to spread a little lime or gypsum down first.
The manure is loaded with a lot of anions, such as phosphorus. The limes have a lot of cations, predominantly calcium.
So we water the manure in afterwards. The manure fills with water, leaches some of it's compounds and hits the lime. Some of these anions and cations attach to one another. It balances out, and helps prevent burning our plant's.
In my opinion exchange capacity really isn't all that necessary to know. As long as your soil has a nice texture, plenty of organic material, and ph is in a good range, it all balances itself out fine.
I always just think of that sponge. Works for me.
Anyway that's my 2cents.
Cheers.
Edit: regarding coco coir, it has a high cation exchange capacity. It naturally holds a lot of cations. This is why a lot of growers like to lime their coir, and supplement calmag etc.
It leaches a lot of cations, but holds on to a lot as well.
It's like a sponge.