Requirements engineering is a discipline within software engineering where model based approaches are still not widespread. Generally requirements are regarded as less or more structured text fragments that are interpreted by stakeholders and developers, who manually manage the requirement interrelationships. We give an overview over the basic concepts of a model-based requirements engineering approach that is currently developed by the project KOGITO. The focus of the paper is on the presentation of the underlying conceptual model and its relationship to different kinds of requirements specification documents. We present an overview over a domain specific conceptual model that is targeted at the requirements engineering for Web-enabled B2B systems. The presented model is capable of managing requirements stemming from various kinds of documents, including B2B standard specifications. Finally, we show how these requirements are integrated and related into our conceptual model. Thereby, general guidelines for the structure of conceptual models for requirements engineering are identified.