When you access an HCM component, say, Job Data, certain values like business unit, company etc are defaulted. What governs this concept?

Values like business unit, company, country get defaulted by virtue of the user’s primary permission list. These default values are associated with SetIDs on the Business Unit Options Default page.
Using the tableset controls and business unit default options, the system determines the default values to enter in select fields on the component. So, when a user accesses a component that uses business unit defaults and selects a business unit, the system determines the setID which drives the defaults for the business unit on the Tableset Control –Record group page.