A Hospital ship provides medical services in the field equal to a full fledged hospital.
Two different terms have been used in the novels with similar meaning. There are no real world Japanese military ships with these classifications in existence. It was not possible to clear up or confirm the differences.
治療艦（ジャースィア）hospital ships (treatment,remedy,relief)(Ghasiac)
衛生艦（ガリュミア）hygienic warship (Garyumiac), perhaps a R&R ship
List of Hospital ship classes