A SB with 6xHosM, some National Guard units (B9.310; assigned to that base); several other bases in the same tactical area; one or more squadrons.
The SB could target itself, increasing its DF value.
The SB could target one of the other bases, increasing its DF value.
The SB could target the National Guard units, raising their DF values accordingly.
The SB could target one of the squadrons (this would apply even if the battle doesn't include the SB itself- typically this would involve a blockade (F7.0)). This is probably the best choice.
A homeworld with 4xSB (each with 6xHosM) and a handful of squadrons facing off against an attacker.
The four SB could each target a separate SQ. This would add 1xDF to all of the ships in those four SQs. If the defender won the battle, the bonuses of D5.20d would impact all four SQs. This is probably the best choice.
The SBs could act in pairs (thus improving the DF of the SC4 by 1, and the larger ships by 2). If the defender won the battle, the bonuses of D5.20d would impact both SQs (but the chances wouldn't be any higher than if one hospital ship targeted each SQ).
The SBs could target themselves. This is probably the worst choice, but there might be strategic reasons to do so.