What exactly is 'conversation_parts'?

I'm trying to set up webhooks (conversation.admin.replied for now) and in the body of webhook i get field `data.item.conversation_parts` and it is a list. I don't why this is a list. Can someone please explain it? What I mean - by every reply I get this webhook and info about this reply is stored in a list, but why? Which cases should I handle where it can be a list with length> 1?