Hi Prashant.
The permission model is for projects or team. This means that users get their permission levels within their project/team (such as editor/viewer). That permissions applies to all artifacts including all their fields for that project/team.
Our current permission model isn't granular to the fields. There isn't a way to associate levels of permissions to specific fields or even specific artifacts. This will be a good enhancement request and you should be encouraged to post it.
There isn't a way to dynamically adjust the artifact's permission differently to owners vs. others. As said, it's statically mapped between a user and each project/team.
Let us know if that helped.
Sagi