The SDF-7 was a flagship that Colonel Jonathan Wolf used to save the Sentinels from the destruction of Praxis. Afterwards the group became renegades. The ship was of the Horizon-T class. (Robotech II: The Sentinels: Into The Fire!)
The SDF-7 was in the universe pre-reboot, thus it may no longer be in continuity.
SDF-7 was a designation used in the Robotech II: The Sentinels novels to identify the new vessel used by the Sentinels after the destruction of the Farrago. Additional ships of the same class were constructed, and were referred to as SDF-7-class ships.
One such SDF-7-class ship named in the novels was the Ark Angel (not to be confused the Angel class ship named the Ark Angel that appears in The Shadow Chronicles). This ship was the one used to rescue the Sentinels from the doomed planet of Praxis. The ship was then used as the Sentinel's flagship for the rest of The Sentinels' novels.
Production model images available on DVD reseases identify the Horizon-T-class shuttle as the SDF-7. This is the base model used in the Sentinels comics, with some variation. The same model sheets identify other ship-of-the-line classes as SDFs as well.