If you want to create a dashboard that will only be accessible from your account, you should create it under the 'Private Dashboard' tab.
We recommend that you work from this space and then if you want to, share it to the brand dashboard tab when it's ready.
If you want to create a dashboard that can be shared with your colleagues, straight away, it should be created under the 'Brand Dashboard' tab.
To sum it up, the only difference between dashboards created in these different spaces is who can access and edit them.
It's always possible to move a dashboard from Private Dashboards to Brand Dashboards and vice versa.
What's a System Dashboard? Read here.