A domain model should be created to get a better understanding of the domain.
Evans, E. (2003). Domain-Driven Design: Tackling Complexity in the Heart of Software (1 edition). Addison-Wesley Professional.
An informal modeling was chosen to avoid the restrictions of other diagram types.
Additional UML-compliant diagrams can be created as the project progresses. However, it was decided that an informal model would be sufficient for initial domain coverage.
The decision was based on the available wireframes and a technical discussion. Wireframes
A simplified Version of the domain model. Domain data model
Further details can be found in the wiki. Wiki
The model was created and further developed in an iterative process.