大乘起信論義記 2.238

2.238

初言同相者，染淨二法同以真如為體，真如以此二法為相，故云同性相。種種瓦器，譬染淨法也。皆同塵性者，器以塵為性也，塵以器為相，故云微塵性相也。

First, as for "the mark of sameness": the two dharmas of defilement and purity both have tathatā as their substance, and tathatā has these two dharmas as its marks; therefore it says, "sharing the nature and marks." The "various earthenware vessels" illustrate the dharmas of defilement and purity. "All sharing the nature of dust" means: the vessels have dust as their nature, and dust has the vessels as its marks; hence it says "the nature and marks of fine dust."