To form an Allegiance (and using the special abilities of that Allegiance) the models of your army must all have the keyword of that Allegiance
(Allegiance dispossessed means that all your models must be dispossessed).
However, you can take allies: with a 1000 pts army, 800 must be dispossessed, and 200 can be ally. With a 2000 pts army, 400 can be allies.
Allies can have a different keyword (thus not dispossessed). Your army will still count as dispossessed Allegiance, and your 800 pts of dispossessed will gain the bonuses of the Allegiance, while the other 200 pts won't gain bonuses.
Every army got only a limited number of allies: for example, dispossessed cannot ally with sylvaneth, even if they are both Order faction.
You can still make an army composed by dispossessed and sylvaneth, but at that point the army won't have the "dispossessed Allegiance", but the "Order Allegiance" (with different, more generic goodies).
Hope it makes sense.