Difference between mod_offline and mod_unavailable

Hi
I want to learn something about difference between mod_offline_post and mod_unavailable_post.

I've succesfully configured mod_interact with ejabberd 16.08 
I want to send mod_offline_post informations(to,from,body, etc...)  to offline users in ejabberd but whenever I send a message to a user which is offline now, mod_interact makes mod_unavailable request

Do you know why?
