The Immutable ID is used to link an on-premise user object to an entra ID user object.

Sidenote, I should really move these snippets to Github

$UserObject = Get-ADUser "CN=User1,OU=Corp.Users,DC=itsrob,DC=local" -server dc1.itsrob.local
$UserObjectGuid = $UserObject.ObjectGUID
$UserObjectGuidBase64 = [System.Convert]::ToBase64String($UserObjectGuid.ToByteArray())
write-host
write-host $UserObjectGuidBase64