KB4090007 is an update which is optional and can be downloaded manually. The update is for Skylake H/S, Skylake U/Y & Skylake U23e for the 6. Generation. You find more details about it here, here or on my Blog, Microsoft itself hasn’t updated their own KB-Article page yet.
Again way too less information given from Intel and Microsoft here, hopefully they are going to update the KB4090007 article to reveal some more details, but I assume they only write down the typically blah which at the end doesn’t say anything. The only thing we for sure know right now is that it’s for Skylake systems.
Microsoft KB-articles are a joke and useless anyway, just my two cents.