Objects cover the things associated with a subject.
Some are literally physical objects:
a guitar
a train
a painting
a telescope
a football
But Object can be interpreted broadly.
A book, song, film, building, company, invention or work of art can all function as Objects within the system.
The question is:
What are we dealing with?
Objects are particularly useful because they frequently act as bridges between otherwise distant subjects.
A guitar can connect a musician to its manufacturer.
A painting can connect an artist to a museum.
A spacecraft can connect an astronaut to a launch site.
A book can connect its writer to a film adaptation.
Objects give links something tangible to travel through.