The rules that use the vocabulary item "class" are:

<ship_type-not_creatable>

<ship_class-not_creatable-sing> class <type_of_ship>

<ship_type-not_creatable-pl>

<ship_class-not_creatable-sing> class <type_of_ship-pl>

<ship_type-creatable-sing>

<ship_class-creatable-sing> class <type_of_ship-sing>

<ship_type-not_creatable-sing>

<ship_class-not_creatable-sing> class <type_of_ship-sing>