俱舍論疏 13.169

13.169

論「又背諸色至伏色想故」，又重釋也。無色界定背諸色故，由彼定能伏色想故，所以無色無無表色。

The commentary states: "Furthermore, turning away from all form ... because it subdues the ideation of form." This is a further explanation. Because the absorptions of the formless realm turn away from all form, and because those absorptions can subdue the ideation of form, the formless realm lacks avijñapti-rūpa.