To define commands for selecting fonts efficiently through a document:
\newfontfamily\<font switch>{<font name>}[<font options>]
\newfontface \<font switch>{<font name>}[<font options>]
To select the default document fonts:
\setmainfont{<font name>}[<font options>]
\setsansfont{<font name>}[<font options>]
\setmonofont{<font name>}[<font options>]
To define an ad hoc font family individually:
\fontspec{<font name>}[<font options>]
To specify features to be used for every subsequently defined font:
\defaultfontfeatures{<default font options>}
\defaultfontfeatures+{<add to default font options>}
To specify features to be used for specific fonts:
\defaultfontfeatures[<font name or switch>]{<default font options>}
\defaultfontfeatures+[<font name or switch>]{<add to defaults>}
To add features to the font family currently in use:
\addfontfeatures{<font options to add>}