瑜伽論記 7.164

7.164

下明作意緣境起九勝解，及狹小所緣意解勝解者，依下結文云：所緣狹少；故此即欲界，若色界名廣大所緣，無量謂無色界。

Below, it clarifies that mental engagement takes an object to generate nine resolved determinations. Regarding "and the resolved determination understanding a narrow object": based on the concluding text below, it says: "the object is narrow and small"; hence this is the desire realm; if it is the form realm, it is called a "broad object"; "immeasurable" refers to the formless realm.