In TeXForm#
TeXForm parses this entry as a character leaf mapped to U+0398 (GREEK CAPITAL LETTER THETA).
\bbalpha\bbbeta\bbchi\bbdelta\bbDelta\bbdotlessi\bbdotlessj\bbepsilon\bbeta\bbgamma\bbGamma\txtbbTheta
\txtbbThetaSymbol
The Θ symbol (Greek capital letter theta, U+0398), written as \txtbbTheta in LaTeX text mode.