俱舍論頌疏論本 10.47

10.47

後二緣欲六者，第三、四禪名為後二，三、四靜慮唯繫六種，彼無六喜，但有六捨，緣欲界境亦得有六，色等六也。

Regarding "the latter two have six cognizing the desire [realm]": the third and fourth dhyānas are called "the latter two". The third and fourth dhyānas are bound to only six kinds: lacking the six joys, they have only six equanimities. In cognizing objects of the desire realm, they can also have six: the six such as form.