An attribute in AutoCAD is an entity, in and of itself, that allows a different text value to be shown for each inserted block reference.
Think of an attribute as enhanced text, that only exists to be used with blocks.
You define the attribute in the block definition.
The block definition contains all objects shown in the block reference.
"Do I
understand correctly that for example, the color inside a circle (or
anything visible as part of the entity) would not be considered an
attribute?"
Correct, these would be properties...
"If so, is any part of the visible entity defined in the block, or
does the block simply point to an entity definition or definitions within
it?"
Yes & Yes
A block definition stores the definitions of objects it should show and the coordinates of these objects in relation to an insertion point.
A block reference uses the definition as a basis for showing the defined list of objects.
You use a block to store the information once in a drawing, then reuse the block by inserting block references into the drawing multiple times, though it is allowed to install a block reference only once.
From the end user perspective, they will only ever see one or more block references, they will not see the block definition.
When a user inserts a block reference into a drawing, they only add a point and a description that says what block should be shown (referenced) at said location.
Ronnie Wilkins, Jr.