Whether to embed the linked resource into the document. If a font isn't embedded you're relying on the same font to be installed on the viewers operating system - if it's not found, an "appropriate" substitution font is used.
If you're going to go to the effort of using a TrueType font, it's usually because you're either a) after a specific style, or b) need characters in the font that aren't in the default fonts. In both cases you should leave set to it's default of "true".
Embedding a TrueType font
<link name="myfont" type="font" subtype="OpenType" src="times.ttf" embed="true" subset="false"/>