A lookup field creates a relationship that links one object to another object. The relationship field allows users to click a lookup icon to select a value from a popup list. The other object is the source of the values in the list1. In this case, the developer should add a lookup field on the Account object to the Global Address object, so that multiple Accounts can share the same Global Address as their default pickup address. This way, the Global Address object can store the common address information for different Accounts, and the Account object can reference the Global Address object via the lookup field. A master-detail field creates a parent-child relationship between two objects, where the master object controls certain behaviors of the detail object. The detail object inherits the sharing and security settings of its master object2. In this case, a master-detail field is not suitable, because the Global Address object should not be dependent on the Account object, or vice versa.
The Global Address object should be able to exist independently of the Account object, and the Account object should be able to have multiple Global Address records associated with it. References:
* Object Relationships Overview | Salesforce Help
* Relationships Among Objects | Salesforce Help