The roles are completely controllable by you. The only thing "fixed" is their order - Editor is above Expert, Administrator is above Editor, etc. So for example if an Expert can do something then Editor/Administrator/Super Administrator can do it too.
You can use all or only some of the roles, for example you can set Experts to be able to edit any post and not use Editors, or allow Editors to do some limited things and not have Experts at all.
Use the Permissions page in the admin area to set all the roles you want.
