Redactor Redactor 3.x Feature Configuration Inline Styles
By adding the inlinestyle button to your toolbar, you get access to a quick selection of inline styles to use.

<mark>for marking parts of the text<code>for inline code references<var><kbd>for (keyboard) shortcuts- superscript (
<sup>) - subscript (
<sub>)
The inline style options can’t be customised. For more flexible inline styles, the custom formatting functionality may be better suited.