俱舍論頌疏論本 27.101

27.101

化二謂欲色，四二外處性者，釋化境也。化有二種：一欲界化、二色界化。欲界化四，謂四外處，色、香、味、觸，名欲界化。色界化二，所謂色、觸，名色界化。以色界中無香、味故。

Regarding "transformation is twofold: of desire and of form; having the nature of four or two external sense fields": this explains the sphere of transformation. Transformation is of two kinds: 1. transformation of the desire realm, and 2. transformation of the form realm. Desire-realm transformation has four, meaning the four external sense fields of visible form, odor, taste, and tangible, which are called desire-realm transformations. Form-realm transformation has two, namely visible form and tangible, which are called form-realm transformations, because there is no odor or taste in the form realm.