Help understanding the i-relation in Lewis’s work on identity.

Hi all, I’m currently reading “Survival and Identity” and I’m stuck trying to understand the i-relation. Specifically what is relating to what? What are the subjects of this relationship? I can see an interpretation in which the continuant and its person stages are the subjects and an interpretation in which the person stages are the subjects only so long as they belong to the continuant (otherwise I suppose they’d just be regular old r-relations). And why does he describe the i-relation as symmetrical? Any insight would be appreciated! I know it’s knit-picky question but it would really help me with the project I’m working on.