A dashboard assigned as the default landing DB for a role
The dashboard(s) a role has access to.
The landing dashboard is but a special type of a dashboard that the role has access to.
A role can have access to multiple dashboards, and a dashboard can be accessible by many roles.
To allow a role to have access to a particular dashboard you should make sure that the user has at least read access to all the components on the dashboard. (modules, actions, modules from which a page selector was published,...etc) This should be done to ALL dashboards a role will have access to and ultimately see in his/her content panel.
Next, a role can have only one landing dashboard, but a dashboard can be assigned as a landing dashboard for multiple roles granted that the roles have been given access to all the dashboard components
This might be more than what you asked for but it's good to be aware of the big picture as well.
Einas "Give a Man a Fish, and You Feed Him for a Day. Teach a Man To Fish, and You Feed Him for a Lifetime"