When I post something, there is no symbol in front of my post..
Some have a blue dot in front of their post and some have a blue pin kind thing…
Why these differences??

Topics that have been newly posted since your last visit, and haven't been read by you are marked with the blue dot and bold text to show that they are unread. Since you have posted this topic and therefore viewed it, it's not marked as unread. It is marked as unread for other people though, including me.

(Try visiting an unread topic, then go back to the list of topics. You'll see that it is no longer marked with the dot as unread )

The blue pin means that the topic is stickied. Stickied topics are topics chosen by the Scratch Team to remain on the top of the topic page. Other topics move down on the page as new topics are created. However, stickied topics will remain on the top as long as they stay stickied.
Correct me if any of this is wrong.