The Small Font class is used liberally throughout vBulletin, and does 'exactly what it says on the tin', that being to specify a style for a smaller-than-normal font.For best results, you should usually specify only a font size and a font family for this class.
Example of element using this class:
<p>This text does not use the Small Font class, <span class="smallfont">but this text <strong>does</strong> use the Small Font class!</span></p>