The items can be devided into different categories depending on their coefficient of rarity.
There are seven of these categories, ranging from the coefficient level 0-6.
The
higher the coefficient level, the
lower the chance of the item spawning in a certain situation is.
So coefficient 0, or C0 for short is something that you already know will spawn with a chance of 100% before it does. Something like

which will spawn when you kill an enemy or

which will always spawn in level 30.
C1 items are very common. Things like

,

or

.
It then gets more and more rare, up to C6, which are insanely rare, like

or the Golded Mushroom.
C4 are items like

or

, which are already considered to be pretty rare.
Chances for a spawned item to be a C1-C6 can be found in the FAQ thread.
