what does quoting do w/ a bash heredoc?

heredoc.sh

#!/bin/sh

greeting="What's up"

# notice `EOF` is not quoted
cat << EOF
    No quotes mean I do var substitution: 

        $greeting
EOF

cat << "EOF"
    
    ------

    "EOF" means no var subtitution
    
        $greeting
EOF


cat << 'EOF'
    
    ------

    'EOF' means no var subtitution
    
        $greeting
EOF

output.txt

    No quotes mean I do var substitution: 

        What's up
    
    ------

    "EOF" means no var subtitution
    
        $greeting
    
    ------

    'EOF' means no var subtitution
    
        $greeting