What the number mean? https://media.forgecdn.net/attachments/428/396/tooltip.png
And the abbreviation?

Alt means Alternate, the number is the phase in which the item becomes available.

And priority goes from top to bottom?

But what alternate really mean? It is bis but not really? It a bit worse than a bis?
Also what "mit" mean? Decypher all abbreviation in the addon description page would be nice.