They are the two levels of Default Trust. EX: DefaultTrust is DT0, people trusted by DT0 are DT1, and people trusted by DT1 are DT2, etc.
I don't get it. Most profiles have 0 trust. Then there are profiles with red trust -x. And profiles with green trust +x. Am I at DT0? When will you enter DT1 and when DT2? When I gain trust, will I immediately be DT1? Do I need 10+ or 100+ green trust to enter DT2? And is there no level DT-1 for profiles with negative trust?