From Gineipaedia, the Legend of Galactic Heroes wiki
These large transport vessels were completely unarmed, and only lightly armored. Functioning solely as a medical treatment facility, these vessels were little more than enormous, mobile hospitals. These vessels were kept at the rear of fleet formations, though they also tended to operate independently of the larger fleets.
Painted white, these ships were typically avoided in firefights, as they were seen as non-combatant vessels.