I see
‘aux-files’ BOOL
If BOOL is ‘#t’, create ‘.tex’, ‘.texi’, and ‘.count’ files when
used with the ‘eps’ backend option. Default: ‘#t’.
‘backend’ SYMBOL
Use SYMBOL as the backend for LilyPond output. Possible values
are:
‘ps’
This is the default setting. PostScript files include TTF,
Type1, and OTF fonts. No ‘subsetting’ of these fonts is done.
Be aware that using ‘oriental’ character sets like Japanese
can lead to very large file sizes.
For PDF output, the ‘ps’ backend is used, too; the resulting
PS data is post-processed by Ghostscript’s ‘ps2pdf’ script,
which also does font subsetting by default.
‘svg’
Scalable Vector Graphics. A single SVG file is created for
every page of output. Music glyphs are encoded as vector
graphics, but text fonts are _not_ embedded in the SVG files.
Any SVG viewer will therefore need the relevant text fonts to
be available to it for proper rendering of both text and
lyrics. It is recommended to not use font ‘lists’ or
‘aliases’ in case an SVG viewer is unable to handle them.
When using _Web Open Font Format_ (WOFF) files the additional
‘-dsvg-woff’ switch is required.
in the advanced options description.
In contrast, lilypond -dhelp states
aux-files (#f) Create .tex, .texi, .count files for use with
lilypond-book
backend ('ps) Select backend. Possible values: 'eps, 'null,
'ps, 'scm, 'svg.
So the list of backends seems to be somewhat inconsistent, and the
default setting of aux-files is documented as #f in the command line
help and as #t in the lilypond-usage manual.