Bell LaPadula model (Bell 1975): The granularity of objects and subjects is not predefined, but the model prescribes simple access rights. Based on simple access restrictions the Bell LaPadula model enforces a discretionary access control policy enhanced with mandatory rules. Applications with rigid confidentiality requirements and without strong integrity requirements may properly be modeled.
These simple rights combined with the mandatory rules of the policy considerably restrict the spectrum of applications which can be appropriately modeled. Source: TIPTON, Hal, (ISC)2, Introduction to the CISSP Exam presentation.
Also check: Proceedings of the IFIP TC11 12th International Conference on Information Security, Samos (Greece), May 1996, On Security Models.