The limits for the values are client side, EQ decides what it will allow through and what it will throw out as a bogus item. It's pretty forgiving though, I've certainly created 100/10 weapons with +50 bane, +50 magic +50 race damage with 50000hp and an Ice comet effect. They show up fine in the Emu.
As to what all of the PI flags mean, well, I don't think anyone this side of VI is sure

Effect is definitely the type of spell effect (Proc/permanent effect/clickable etc.), Spell is the spdat ID of the spell on an item. Count and Countflag I'm not sure of, I just know that when Countflag isn't 2 that the client seems to forget that items are nodrop or have spell effects on. Most of them are duplicates of other values, so it's possible that they are used at runtime on the client to store temporary information, though that's just a guess.
I'll have a look at the item creation in my editor, I coded that feature in about 5 mins so it's not undergone the most rigorous of testing. An item number of -26000 doesn't sound great
K.