It's pretty easy to find the subjects in these declarative sentences.
The fall is a beautiful time of year.
Water freezes at 32 degrees Fahrenheit.
It's a bit harder to find the subjects in commands. Can you find the subjects in the two example sentences?
Walk the dog.
Wear a warm coat.
The subject in imperative sentences is implied, and every single imperative sentence has the same implied subject: you.
(you) Walk the dog.
(you) Wear a warm coat.
Because this you is implied - or understood - we call it understood you.
I realize that this sounds kind of funny, but when you think about it, it makes sense.
Whenever you give a command, you are addressing someone or something. How would you be able to give the command if you weren't addressing them?
Let's get back to Trish's question. Trish is wondering about commands that begin with a noun of direct address.
Bob, walk the dog.
Cathy, wear a warm coat.
We call the words Bob and Cathy nouns of direct address because they are addressing the person whom we are speaking to.
In Trish's question, she refers to these nouns as "stated subjects" because Bob and Cathy seem to be the subjects. However, Bob and Cathy are not subjects.
Remember that the subject of all imperative sentences is (you). This remains true even when we begin an imperative sentence with a noun of direct address.
Bob, (you) walk the dog.
Cathy, (you) wear a warm coat.
It is a coincidence that in these cases the noun of direct address and the subject (you) are referring to the same person. However, that does not mean that you can use them interchangeably as subjects.
Does it still seem like nouns of direct address should be subjects? If so, here is something helpful to consider.
Check out these declarative sentences that begin with nouns of direct address.
Tom, the fall is a beautiful time of year.
Jerry, water freezes at 32 degrees Fahrenheit.
Yikes! If we treated nouns of direct address as subjects, we wouldn't know if the subjects were Tom and Jerry or fall and water.
Luckily, we don't have to worry about this because nouns of direct address are never subjects.