For a three layered application model, in some cases, the UI interface layer can use the DAL directly, rather using it via the BLL (i.e. “immediate next layer”). This type of application model generally includes a small set of classes and methods in business logic layer, ignoring the mapping all functionalities from DAL to BLL. This design issue provides a sort of performance, by reducing the engagement of intermediate layer(s).
On the other hand, we can have a model, where a complete set of functionalities is needed to be present in BLL, is “tightly coupled”. In this context UI layers can’t use the functions of DAL directly.
In this point, the software architect is needed to address this design issue clearly.