Different implementations have elected to use different syntaxes for specifying subdirectory searching. In the interest of typographic clarity, the examples here do not use the replaceable font.
via a separate TEXFONTS_SUBDIR environment variable.
t:\edirectory!!; t:\edirectory! for a single level of searching.
texmf/subdir//
texmf:[directory...]
texmf/subdir/**; texmf/subdir/* for a single level of searching.
t:/directory// or t:\edirectory\e\e.