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Refers to the calligraphic font accessed via \mathcal. The default font representation looks like this:

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\documentclass{article}
\pagestyle{empty}
\begin{document}
$\mathcal{A\ B\ C\ D\ E\ F\ G\ H\ J\ K\ L\ M\ N\ O\ P\ Q\ R\ S\ T\ U\ V\ W\ X\ Y\ Z}$
\end{document}

Although, other representations of \mathcal also exist (see Comprehensive LaTeX Symbol List, Table 213):

enter image description here

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