In TeXForm#
TeXForm parses this entry as a character leaf mapped to U+03C3 (GREEK SMALL LETTER SIGMA).
\bbalpha\bbbeta\bbchi\bbdelta\bbDelta\bbdotlessi\bbdotlessj\bbepsilon\bbeta\bbgamma\bbGamma\bfbbsigma
\bfbbsigmaSymbol
The σ symbol (Greek small letter sigma, U+03C3), written as \bfbbsigma in LaTeX math mode.