Imagine classifying a picture of a starving child. The 'about-ness' of the picture is starvation, hunger, poverty, etc. The 'of-ness' is that is of a child. The 'thing-ness' is that it is a picture.
'About-ness' is the most difficult to classify because it is subjective. Folksonomy is often about 'about-ness.'
A general rule of subject analysis is to index to the level of specificity of the item; however, the end user generally searches by broader terms because they underestimate the specificity of analysis.