Above is an Item Rank Chart for a majority of the items in Disgaea 5. Item Rank is a hidden statistic on items that determines a few things, with one example being the level of enemies you will encounter when going into its Item World. The red numbers on the chart above are the base levels of enemies for the normal items when entering Item World. For example, going into an Ultimus at level 0 and no enemy strength stars will have the enemies appearing at around level 171.
Another thing that the Item Rank does is determine the kind of items and equipment you will find on the Bonus Gauge and equipped to enemies. You will find items around the same rank, with higher ranked items appearing in the Bonus Gauge and on enemies as you delve deeper. By leveling an item to 80 or more before reaching floor 100, you will find an Item God 2 on that particular floor, who will have the next rank item equipped (i.e. an Item God 2 in a Yoshitsuna will have the Baal Sword equipped).
One last thing that needs to be talked about in regards to the Item Rank Chart above is the Rank 40 weapons. Unlike the previous games, there are now three different Rank 40 weapons, each of which have different ways in obtaining them. The first one (marked with a * on the chart) is gotten by diving into the Rank 39 weapon and leveling it to 80+, then stealing it from the Item God 2. The second () can be acquired by researching a Rank 40 Netherworld, then defeating the Overlord once it’s been discovered. The last one (*) requires you to find the Legendary Weapon mystery room in the Item World, which is completely random. If you have a character with a Weapon Mastery of 40 (normal) or 100 (Carnage equivalent), then talk to the weapon to receive the corresponding weapon of that type (Fist if you’re in a Fist’s Item World).