Often in ones attempt to clarify, more confusion is created. I hope that will not be the way this post goes. Read on.
Definitions in the visual arts field are not always consistent and various artists choose to personalize terms based on their understanding of a word. I'm no exception. I was once sternly corrected for using subject and content interchangeably when these two words have discrete meanings according to my knowledgeable friend who went to art school (I didn't, but I can research!).
Subject matter is the literal, visible image in a work while content includes the connotative, symbolic, and suggestive aspects of the image. The subject matter is the subject of the artwork, e.g., still life, portrait, landscape etc. Gerald Brommer in Emotional content: How to create paintings that communicate notes that "Content is the reason for making a painting." He further elaborates:
"Content is not subject or things in the painting. Content is the communication of ideas, feelings and reactions connected with the subject...... When we look at a painting its content is what is sensed rather than what can be analyzed. It is the ultimate reason for creating art." Something in the painting must appeal or speak to the heart, spirit and soul of the viewer. He specifically calls this "emotional content".
If you refer back to my first and second post of the Compose series, you will see that my use of the word subject also inlcludes the characteristics of content noted above. I guess that is why I am comfortable using subject to cover everything.
Here's an image I posted before from my "Reading a Garden" series. The subject matter is a still life consisting of a gate with leaves behind it. What is the content?