What does the number mean on the top right column of armor?

[fugazi70]

For instance my robe says +6 but if I take it off, I don't see my ac go up or down..

So what does the top right number mean?

[Sir_Chonas]

overall enchantment level of the item

If it is a +1 enchantment weapon of +2 shatter then it'll be a +3 . . . or something like that. Basically you want that number to be as high as possible while the number of actual enhancement modifiers to the item (shatter, deathblock, whatever) is as low as possible. A +6 stat item is worth more than a +5 stat item of spearblock (or something of that nature).

[Catholicon]

So for example, a "Lightning Guard Robe" will be +1 and a "Lightning Guard Robe of Lesser Acid Resistance" will be +2 or something like that?

[Bronko]

So for example, a "Lightning Guard Robe" will be +1 and a "Lightning Guard Robe of Lesser Acid Resistance" will be +2 or something like that?

That number on the top-right doesn't really mean anything in regards to armor. All weapons and armor have one. It is kind of a loose measure of the relative 'strength' of the item based on enhancement, traits, and abilities. But again: that number is meaningless in almost all cases.

The only time that number has any value is in The Shroud raid. In that raid certain depleted energy cells can be 'powered up' by inserting the cell into the corresponding altar along with a weapon of the appropriate strength. The charged energy cells are then used in crafting recipes. The '+' number in the top right corner indicates whether or not that weapon will work in the powering up process or not. Armor cannot be used for this process.

If I recall correctly the high energy cells require a weapon with a +9 or better rating in the top right corner, the medium energy cells require a +7 or better rating, and the low energy cells will work with just about anything. Just as a forewaring before you start crafting: any weapon you place into the altar to power up a depeleted cell gets consumed in the process and is gone forever.

For a more detailed description on Shroud crafting please see the DDO Wiki entry for power cells or Cambo's highly rated Stone of Change - Eldritch Rituals Cookbook.