While you are revising your Item, You specify XML fields of 2 types.
- 1.Unique identifiers to the Item. Fields like SKU / ItemID fall in this category
- 2. Fields to be revised. Ex: Title, Description etc.
If the Item is already managed by SKU, Then we specify the SKU and Item ID to revise any other fields. It's just like ItemdID specified in the ReviseFixedPriceItem call, Which is placed in the request to revise other fields but not to revise ItemID itself. In other words that documentation refers to Items that are already managed by SKU, a.k.a listed with Manage By SKU.