How is runTime object also the objectRegistry?
 

From what I have read I have come to understand that a runTime object is an object of class Time. How is runTime object also used as the objectRegistry?

The class Time is derived from the class objectRegistry. Hence, the runTime object is an objectRegistry. It certainly makes sense, since a simulation might feature one or many meshes, but there can only be one runTime. [The Highlander pun was definitely intended.]

Attached, you find a class diagram which shows a part of the ancrestral tree.