What does D1 stand for?

First Dorsal Nerve - Second dorsal nerve is D_{2, etc., through D12} - Medicine (DAVI)

First Dorsal Vertebra - Second dorsal vertebra is D₂ , etc. - Medicine

Day One - First day seen for treatment - Medicine (MELL)

Double First Class

First Coast Guard District - Boston, MA - USCG (TAG)

Slight Damage - Emergency Management (EMA)