So it looks like Calc Volume only considers the first parent object (in the total hierarchy), if NO objects are selected.
It also only considers a single object, (the first in the hierarchy) if multiples are selected.
It also does NOT consider selected children.
Below are 4 example pics.
In the 1st, its only calculating the volume of the first top-level parent, and not its children, either.
ie, it only calcs the vol. of the 1m Cube parent, on the bottom left.
In the 2nd, again, its only calculating the top-level parent, again only the 1m Cube object.
In this 3rd, you can see the originally, singly selected object was the Sphere.001, then after adding the objects Cube and Cube.001 to the selection, it only calcualtes for the first originally selected parent.
In the 4th, its again only calculating the first parent. In this example, the 2m Cube.001 object.
So it looks like Calc Volume only considers the first parent object (in the total hierarchy), if NO objects are selected.
It also only considers a single object, (the first in the hierarchy) if multiples are selected.
It also does NOT consider selected children.
Below are 4 example pics. In the 1st, its only calculating the volume of the first top-level parent, and not its children, either. ie, it only calcs the vol. of the 1m Cube parent, on the bottom left.
In the 2nd, again, its only calculating the top-level parent, again only the 1m Cube object.
In this 3rd, you can see the originally, singly selected object was the Sphere.001, then after adding the objects Cube and Cube.001 to the selection, it only calcualtes for the first originally selected parent.
In the 4th, its again only calculating the first parent. In this example, the 2m Cube.001 object.