Item group is a very broad object. It involves a lot of reporting and functionality around the solution (Like forcasting and vendor statistics), but for us it is most importantly the primary grouping for the inventory posting setup. Most commonly the inventory posting setup is similar for many item groups. It is a hassle to maintain the posting setup for all item groups if you use the item grouping that is normal for a company. Doing something wrong in the posting setup could have pretty big ramifications. So we've ended up with having a item groups like: Finished goods, Semi finished goods, Raw materials, Services and so on. I would love to see a separation between the Item group create a new Item Posting group.