DefaultTrust is automatically set to trust level 0 on every user's profiles until they remove it.
DefaultTrust contains some users in its trust list:
theymos
HostFat
dooglus
Maged
dserrano5
OgNasty
Tomatocage
SaltySpitoon
philipma1957
Cyrus
Blazed
hilariousandco
OldScammerTag
These users are considered DT1. DT1 members can give trusted feedback to user profiles, since they are in trust depth 1 of normal users' trust lists.
Additionally, DT1 members can add other users to their trust list. These such users are considered DT2. They can also give trusted feedback to user profiles.
By default, one's trust depth is set at 2. Thus, DT3 members' feedback will not typically appear to be trusted on a user's trust profile.
Don't trust DT1/DT2 all the time, though. Make informed decisions.