Whether to unembed embedded fonts that do not have an obvious
substitute on the local machine.
The difficulty of providing an appropriate substitute font
depends greatly on how unusual the font is. Subsitutes are
typically rather normal fonts so if you have a font like "Transport
Medium" (the font used for all UK road signs) then a PDF display
package should be able to provide a fairly sensible substitute.
However if you have a font like "Fire and Ice" it is likely that
the substitute will not be terribly appropriate. The most extreme
example of this comes with barcode fonts which of course look
completely different from the characters that they represent.
If is difficult to know what fonts are unusual. However if the
font does not appear on the local machine then it is a reasonable
assumption that it may not substitute well. As such this property
allows you to avoid unembedding such fonts.