Dear Aaron...
A \tag is not a location per se, but rather it marks the following bit
of music with some identifier.
[.....]
Thanks a lot for your examples, now it's perfectly clear!
I post here some other examples inspired by your code. I did my best to write the easiest as possible examples. I hope they will be useful to others. The best would be to see them beside a screenshot of score generated (I don't put the screenshot here because of the mailing list policy).
% appendTag {} examples
\version "2.19.83"
someMusic = \fixed c' {c1 \tag #'first {} c'}
{\someMusic}
{\appendToTag #'first e'4 \appendToTag #'first f' \appendToTag #'first g' \appendToTag #'first a' \someMusic}
{\appendToTag #'first {e' f' g' a'} \someMusic}
% pushTag {} examples
\version "2.19.83"
someMusic = \fixed c' {c1 \tag #'first {} c'}
{\someMusic}
{\pushToTag #'first e'4 \pushToTag #'first f' \pushToTag #'first g' \pushToTag #'first a' \someMusic}
{\pushToTag #'first {e'4 f' g' a'} \someMusic}
% appendTag & pushTag {d2 e} examples
\version "2.19.83"
someMusic = \fixed c' {c1 \tag #'first { d2 e} c'1}
{\someMusic}
{\appendToTag #'first f'4 \appendToTag #'first g' \appendToTag #'first a' \appendToTag #'first b' \someMusic}
{\appendToTag #'first {f' g' a' b'} \someMusic}
{\pushToTag #'first f' \pushToTag #'first g' \pushToTag #'first a' \pushToTag #'first b' \someMusic}
{\pushToTag #'first {f' g' a' b'} \someMusic}
% appendTag & pushTag <>
\version "2.19.83"
someMusic = \fixed c' {r4 \tag #'first <> r4}
{\someMusic}
{\appendToTag #'first e'2 \appendToTag #'first g' \appendToTag #'first b' \someMusic}
% {\appendToTag #'first <e'2 g' b'> \someMusic} % wrong, doesn't compile
% {\appendToTag #'first {e'2 g' b'} \someMusic} % wrong, doesn't compile
{\pushToTag #'first e'2 \pushToTag #'first g' \pushToTag #'first b'\someMusic}
% {\pushToTag #'first <e'2 g' b'> \someMusic} % wrong, doesn't compile
% {\pushToTag #'first {e'2 g' b'} \someMusic} % wrong, doesn't compile
{\appendToTag #'first e'2 \appendToTag #'first g'4 \appendToTag #'first b'4 \someMusic}
{\pushToTag #'first e'2 \pushToTag #'first g'4 \pushToTag #'first b'4 \someMusic}
% appendTag & pushTag <e>
\version "2.19.83"
someMusic = \fixed c' {r4 \tag #'first <e>2 r4}
{\someMusic}
{\appendToTag #'first g' \appendToTag #'first b' \someMusic}
{\pushToTag #'first g' \pushToTag #'first b' \someMusic}
% appendTag & pushTag <e> others examples
\version "2.19.83"
someMusic = \fixed c' {\tag #'first <c e g> \tag #'first <c e> \tag #'first <c> \tag #'first <>}
{\someMusic}
{\appendToTag #'first b' \appendToTag #'first d'' \someMusic}
{\pushToTag #'first b' \pushToTag #'first d'' \someMusic}
% appendTag & pushTag <<>>
\version "2.19.83"
someMusic = \tag #'here <<>>
{\someMusic} %this doesn't generate any score
\pushToTag #'here {c'2 d'} \pushToTag #'here {e'2 f'} \someMusic
\appendToTag #'here {c'2 d'} \appendToTag #'here {e'2 f'} \someMusic
% appendTag & pushTag <<g'1>>
\version "2.19.83"
someMusic = \tag #'here <<g'1>>
{\someMusic}
\pushToTag #'here {c'2 d'} \pushToTag #'here {e'2 f'} \someMusic
\appendToTag #'here {c'2 d'} \appendToTag #'here {e'2 f'} \someMusic