<Prereq>
                <Type>Race</Type>
                <Attribute>Race_Type_Wraiths</Attribute>
            </Prereq>

for race-types. In order for a trait to appear, all the prereqs must be fulfilled. If you have two different races you'll never see it.

If you specifically want to exclude only one unit or race, you can prereq a unitstat at 0, then set the unitstat to -1 for those units you wish to exclude. (edit, I haven't actually tried that, but it should work)

I believe that to be the case, but you can TECHNICALLY add excluding prereqs using the method I described.

            <Prereq>
                <Type>UnitStat</Type>
                <Attribute>UnitStat_BG_HasInjuries</Attribute>
                <Value>0</Value>
            </Prereq>

All units start with this stat at 0. So every unit can get it. But if one unit starts with this stat at -1:
<UnitStat_BG_HasInjuries>-1</UnitStat_BG_HasInjuries>

then he can't get it and it won't appear. I tested it this time.

When you put that prereq in, it checks to see if your faction is that racetype. It is not a check against the individual's racetype. In other words, an Altarian faction may recruit a Mancer hero who would still gain the trait.

Oh, you're talking about the screen where we create custom sovereigns? Yeah, I could see some complications arising there. I was talking about level-up traits. The custom sov screen most likely was not set up to accommodate the prereq thing

Well now... you can create custom allegiances! That changes things.