Go on Setup > Custom settings and select "Country_Regions".
Open the country name you are interested in. You will see all information related to the region:
Now, let's take an example. Open a sales quote with "Region restricted" as visibility. You will have to go on the account page to check the country. Then, go on the setup to check the "Region (Security)" field. Compare the region of the user and the region of the account to know if it is normal that the user can or cannot access some information.