It seems the stratification of the texts is about the formation of strata in the EBTs. If so, it may be connected with the issue of angas (classifications) in EBTs. The following article by Choong Mun-keat may be useful:
“Ācāriya Buddhaghosa and Master Yinshun 印順 on the Three-aṅga Structure of Early Buddhist Texts”, Research on the Saṃyukta-āgama (Dharma Drum Institute of Liberal Arts, Research Series 8; edited by Dhammadinnā), Taiwan: Dharma Drum Corporation, August 2020, pp. 883-932.
Or this site: