俱舍論記 1.229

1.229

「心心所法至彼作用義」者，此釋處義。心等生長門義是處義。彼十二種，能生長心等，故名為處。法體先有，不可云生，但能生長彼作用義。

"Mind and mental factors ... the meaning of their functioning" explains the meaning of āyatana. The meaning of a gateway for the arising and growth of mind and the rest is the meaning of āyatana. Those twelve kinds can cause mind and the rest to arise and grow; hence they are named āyatanas. The entity of a dharma exists beforehand and cannot be said to arise; they merely cause the functioning of that [dharma] to arise and grow.