What does the UM stand for in the in the Salesforce instance naming? EU: Europe NA: North America UM:????
Is it new or has it always been present? Never noticed it and I have just found out that my instance will be there in January
It refer to London, UK (North) / London, UK (West).
As per docs: We will soon have two geographically dispersed data centers in the London, UK metropolitan area - London, UK (North) and London, UK (West).
From a SF slide deck "Tech Updates December 2018 Edition":
"Going forward, we will have two separate subregions within the EMEA region: UMEA (UK, Middle East & Africa) and EU (Rest of Europe & Russia) with UM and EU instances, respectively. UM instances will be supported by two data centers in the London, UK metro area and EU instances will be supported by our France & Germany data centers. "
Full overview of instances and location, as already mentioned in another answer, see https://help.salesforce.com/articleView?id=000257307&language=en_US&type=1