There are no commands for selecting symbol fonts. Instead, these are
selected indirectly through symbol commands like \oplus.
Section 3.6 explains how to define symbol commands.
\mathnormal{<math>}
\mathcal{<math>}
\mathrm{<math>}
\mathbf{<math>}
\mathsf{<math>}
\mathit{<math>}
\mathtt{<math>}
Each math alphabet is a command which can only be used inside math
mode. For example, $x + \mathsf{y} + \mathcal{Z}$ produces 
.
\mathversion{<version-name>}
This command selects a math version; it can only be used outside math
mode. For example, \boldmath is defined to be \mathversion{bold}.