Models have "business" level of abstraction when they do not refer to computer systems at all. Business Level models capture the essence of the business, thus providing full freedom in selecting the optimal IT solution. TOGAF Business Architecture and MDA Computation-Independent Model can be cited as examples of models done and business level of abstraction. The challenge of business-level modelling is to provide sufficient level of details without "contaminating" the model with system-specific details.
Functional Level models include Models that application behavior of a system or integration between multiple systems in platform-independent manner. MDA Platform-Independent Model, Logical Data Model and traditional Use Cases are the examples of Models that have "functional" level of abstraction.
Technical Level contains the Models that would be called Platform-Specific Models under MDA or Physical Models in Database Design. Technical models have direct and preferably automatic connection with software code.
Requirements Model is a Collection of Requirements Statements that are not allocated to specific level of abstraction (say, Business) nor to a specific paradigm (say, Information Structure). Requirements Model is required hen some of the Requirements are provided by an external party in form of plain text or excel spreadsheets.