It starts with the game, on Blizzard's servers. From my understanding, the servers will process it just like it would an ingame macro. Say you have a macro like this

Code:
/assist party1
/castsequence spell1, spell2, spell3
/cast something else
/say Whooties!!
/click actionbutton#
/swapactionbar 2
Blizzard's servers receives all that information at once when you press the macro and processes it from the top down, taking any action that {according to its coding/rules} it is able to process and neglecting anything else until the next time it is sent. When it is sent again, it starts right back at the top looking for the first thing it can process.

The priority system works on the exact same principal, but rather than sending a macro with a ton of commands, we're sending a list of keys

Code:
<Key 1>
<Key 2>
<Key 3>
<Key 4>
Again, Blizzard's servers receives all the keys and just like the macro it goes from the top down, calling the associated macros/abilities in order until it has processed everything it can according to its' rules.


Outside of the order we put the keys in our list, all the "decisions" are made by and on Blizzard's servers, according to their rules.

Think of it like sending Blizzard a box of groceries.

We can pack the box putting the bread on top, a jar of peanut butter underneath that, maybe an assortment of jams under that, perhaps a jar of Nutella and a YooHoo on the bottom. Blizzard gets the whole package at the same time, and will have to unpack it in the same order {bread first, YooHoo last - 'cuz that's the way Mama Bliz taught them to do things}.

Maybe they get excited and Blizzard says "Awesome! Bread!" and starts eating slices of bread, ignoring everything else in the box. Maybe they aren't totally consumed by the bread because it's full of stuff off the GCD and doesn't need two hands to eat, so they use their free hand to keep unpacking the box, and they come across the peanut butter.

Now they need to stop and put the peanut butter on the bread. Or maybe not. Maybe they just had peanut butter so now they're on peanut butter cooldown.

So they set it aside and come across the jams. And one by one they look at the kinds of jams they have, perhaps sampling them in order. Unless they're also on jam cooldown and decide to keep digging through the package.

And they come across the Nutella, and maybe they slap that on some of their bread, unless they have a strict rule about Nutella where they can only eat it for dessert {think Execute or proc conditions } or perhaps they can't resist Nutella and have have it all the time 'cuz Nutella is never on cooldown.

But maybe, just maybe, they don't want any of that stuff and all they're left with is a YooHoo at the bottom of the box. But with all that bread, they sure could use a drink, so they put everything else away and sit down with their Yoohoo and their bread and wait for the next package we send, which they'll still have to unpack bread first, but maybe they're on bread cooldown by then...

I think that makes the general point, lol.




Yeah, I'm just a little hungry.