If you ask developers about interfaces they typically build you will hear some form of create, read, update, and delete (CRUD). CRUD-type interfaces are simple to understand and are intuitive. They tend to be used heavily in the data access layer operating on your various data entities. At a high level these interfaces seem to meet your product line needs. This may not always be the case though! Why do I say this? The interfaces you create for a product line depends on the variations you need to support and manage. Variations will require interfaces that vary in granularity. Some operations might operate on a data entity while others might operate on an aggregate. Still others might operate at a subset of a data entity. For example, say you develop solutions for the banking product line where in you have an online portal, mobile bank, and branch services. All these products in your product line might support money transfer. Online portal might display a complete set of from and to accounts while mobile might display only a primary account. Branch might provide only accounts eligible when initiated by a branch. To address the variations you have to go beyond just getting the right set of accounts and have a reusable asset use a mechanism to access and execute business rules.
Create interfaces from a product line context