Post on leofinance, hive.blog or any other frontend are posts, and they are eligible for votes and rewards distribution.
Custom json is also basically a text and you can store whatever text inside those.
The main difference between custom json and posts/comments is that jsons are not eligable for votes .... they are like a side nodes on the blockchain.
Votes on hive-engine tokens are not json. HE tokens transfers are.
The voting mechanics on the Hive engine is called scotbot (basically trails the Hive votes) and that one is the only closed source softwere that Hive Engine has. The other is open.
Thanks, that clarifies many things.