what is the difference between subscribers and posters if both can send to the group. 

Yaso, a subscriber is someone who receives the messages via the mailing list. The poster can submit messages to the mailing list. Andrea has referenced the correct setting, and configuring these values should resolve your query. 

In addition to Kyle's reply... Let's say you wanted to allow only the Moderator and a select number of subscribers to post to the mailing list. In the Allowed Posters setting, you'd choose Moderator. You would then add the additional subscribers to the Posters tab. On the other hand, if all subscribers should be allowed to post to the mailing list, and the Moderator is a subscriber on the list, you will simply need to change your Allowed Posters setting to Subscribers Only. 

Now it is very clear. maybe in future better to name it as "senders" or "permissions"  send only / receive only

however appreciate your support and work

great work.