At this point, you, in the role of the spec writer, actually enter the realmof deciding the user experience. And, needless to say, you will need allyour wits about you to successfully complete this process - a mistake herecould have significant repercussions on the user's perceptions of theapplication's ease of use.
In this critical phase, models can play an important role in designing,developing and testing assumptions and ideas about how the applicationshould be structured. Essentially, a model is a structure diagram that helpsthe writer depict application components and the features included in each.Models may be defined from the point of view of both the user and thedeveloper: a user-based model is the user's conceptual view of theapplication and is guided by user research and studies, while adeveloper-based model focuses more on the architecture and data structure -modules, programming interfaces, logical relationships et al - of theapplication.
You can also consider representing information using diagrams and flowcharts- for example, data flow diagrams, which indicate how data is stored andprocessed by the application, and object interaction diagrams, which providean overview of the objects that make up the application and theirinteraction. In case your application uses a database, you might alsoconsider including one or more entity relationship diagrams to represent thedesign of your database.
An important thing to note here is that, in the garb of the user experience,you are more or less structuring the application. It is therefore essentialfor you to talk to the development team in detail about the constraints andassumptions that they will be working with. A detailed analysis of this kindsaves the entire team a lot of wasted time and frustration in later stages.