In TeXForm#
TeXForm parses this entry as a character leaf mapped to U+03A3 (GREEK CAPITAL LETTER SIGMA).
\bbalpha\bbbeta\bbchi\bbdelta\bbDelta\bbdotlessi\bbdotlessj\bbepsilon\bbeta\bbgamma\bbGamma\txtbbSigma
\txtbbSigmaSymbol
The Σ symbol (Greek capital letter sigma, U+03A3), written as \txtbbSigma in LaTeX text mode.