Good question. COTS items are a subset of commercial items and are generally acquired using the same FAR Part 12 procedures that “non-COTS” commercial items are. However, COTS items are not typically subject to modification whereas commercial items can be modified and not lose their "commercial item" status. There can be differences between how COTS and non-COTS commercial items are treated. For example, the clause at DFARS 252.225-7009 (Restriction on Acquisition of Certain Articles Containing Specialty Metals) contains exceptions in para. (c) of that clause which don't apply to non-COTS commercial items.