An object type can be a main object type, a sub object type or both.

Object typeMain or Sub
Column*) Main
*) Sub (Document library / List)
Document library*) Main
Group*) Main
List*) Main
Permission*) Main
*) Sub (Document library / List)
Permission level*) Main
View*) Sub (Document library / List)

A main object is defined on site level. A sub object needs a main object to be defined on.

From a SharePoint perspective, can columns and permission also be defined on site level. This is not on the roadmap and therefor strikethrough.

SPOT does not support sub sites! Only top-level sites can be used.