When does mail contact show as 'online' in chat or contact list?

I'm confused..one of my yahoo contacts shows as available in mail chat, but doesn't show online in the contact list. Another contact shows as online in the list, yet doesn't show up as available in mail chat. What's the difference? When the one contact shows as 'available' in mail chat, does that mean he is chatting or just online on yahool? If he is chatting with messenger, will he show as online in the yahoo contact list? Thank you for clearing this up.