IDMSA (full name idmsa.apple.com) is the name of the guard shack placed at the network security fence built around Apple and Apple services, and you will have to present your identification badge and your secret passcode to the guard to be permitted access into parts within the fenced perimeter at Apple.
IDMSA is what you authenticate to.
In more technical jargon, IDMSA a cryptic acronym used as the collective name of the group of computer servers — computers larger, faster, and with more resources and with capabilities necessary for handling the billions of network connections that can be happening — used to provide authentication services for controlling and accessing Apple apps and services. That includes the authentication services used to allow (or to deny) access into the Apple Support Communities.
Much like the network host names for DuckDuckGo.com or Qwant.com or some other search engine, or like how you have assigned names to your own local computers and phones and printers and other devices on your own network, many things connected to the network have network host name. The (immense) Apple firewall and authentication services implementation has a host name too, and that name is IDMSA. That name spelled out is idmsa.apple.com
IDMSA is involved with any use of Apple services, Mac or iPhone or iPad or Apple TV 4K or otherwise.
