Objects are a high-level abstract tag, meaning they serve as a base for many other tag types but cannot be directly created themselves. Generally, they are "things" with a position in the world but are distinct from the "level" itself. Some examples include elevators, trees, warthogs, and the player.
Some capabilities available to objects (though not used by every subtype) are:
The radius should contain this object completely, but not be too large. Setting it too high will cause the object to draw unccessarily when off-screen, but setting it too low will cause the object to have clipped shadows and pop in and out of visibility at the edges of the screen.