This is a "catch-all" for anything strange in your background. For example, if you traveled extensively as a child and claim to have several "native" languages, this would work. Similarly, if you have access to a skill that people around you don't have, this is unusual. In general, if a reasonable background gives a character something unusual, the GM can allow it but require this advantage to pay for it. Stranger backgrounds may require higher penalties, such as "Wookiee raised by Imperial officers" or "trained from birth by secluded Force-using ninjas."