Use cases were introduced into the Unified Modeling Language to capture the functional requirements in object oriented systems development. This work reports the results of comparing two controlled experiments conducted on use case models with different subjects, in which the effect of use case format on users understanding of systems requirements is assessed. Replication with subjects of different knowledge in use case technique allowed us to investigate whether subjects experience play any role in the comprehension of use case models. The results of the controlled experiments showed that for the comprehension tasks, which required only surface understanding of the Use Case model, the provision of diagrams along with the textual use case descriptions significantly improved comprehension performance of both novice and high knowledge users. However, diagrams had no effect on users performance in the deep understanding tasks. Moreover, there was no evidence that prior experience with use cases has influenced subject's performance in surface and deep understanding tasks in familiar and unfamiliar application domains.