How do I know which 'Organization hierarchy' scope to select for Area of Responsibility?

For person security profile, if you choose Organization hierarchy for department as scope, you get access to people who are working in department present in subbranch of the organization tree, which the signed in user is responsible for.

If you choose Organization hierarchy for legal employer as scope, you get access to people who are working in legal employer present in the subbranch of the organization tree, which the signed in user is responsible for.

For position security profile, if you choose Organization hierarchy for department as scope, you get access to positions from departments present in subbranch of the organization tree, which the signed in user is responsible for. Alternatively, Organization hierarchy for business unit grants access to positions from business units present in the subbranch of your organization tree, which the signed in user is responsible for.