大乘起信論義記 2.165

2.165

謂與眾生作六根境界故。《寶性論》云「諸佛如來身，如虛空無相，為諸勝智者，作六根境界，示現微妙色，出於妙音聲，令嗅佛戒香，與佛妙法味，使覺三昧觸，令知深妙法。」故名妙境界也。

This means producing the realms of the six sense faculties for sentient beings. The Ratnagotravibhāga says: 'The bodies of the Buddha-Tathāgatas / are signless like empty space; / for those possessing superior wisdom, / they produce the realms of the six sense faculties, / manifesting subtle and wonderful forms, / uttering wondrous sounds, / causing them to smell the fragrance of the Buddha's precepts, / granting the taste of the Buddha's wonderful Dharma, / causing them to feel the tactile sensation of samādhi, / and causing them to know the profound and wondrous Dharma.' Therefore, they are called wonderful sense realms.