Prefix/suffix mod levels?

I'm a bit confused. In the prefix/suffix mods each one has a level, the higher the mod value the higher the level. What is this level dependent on. The level requirement of the item? Am I right in saying a mod value can only go as high so that its mod level is equal to or below the item level requirement?

http://www.pathofexile.com/item-data/

Check on here. The lvl on Armor/Weapons are the minumum level of mobs that can drop them. However. mobs lvl 60 can still drop a lvl 20 weapon.

In that case, the lvl requirement for that weapon would be 20. However the hidden ilvl of that item will still be 60 (from the mobs that dropped it).

Then take a look at the prefix/suffix data. The lvl in those column are the minimum ilvl required for it to be able to show up. (Some affix can only spawn on specific item type, and some are not in there as the table is not always complete).

This can result in a blue lvl 20 base sword with ilvl 60 that has a lvl 50 affix. The lvl requirement for that sword will be the higher number between 20 (base lvl) and 40 (80% of highest lvl affix).

(Speculation on the last part, but the general idea is that high lvl affix will raise the item required lvl, but still lower than the higest lvl affix)

What if the item is bought from a vendor?

They go up with the difficulty/act the vendor is in, you can check hidden ilevels by picking the item up and then typing /itemlevel into the chat pane.