Designing functional specifications is a critical step in system engineering.

Funcional Specifications are sometimes pictured by means of informal interface prototypes showing User-System interaction and enhanced with comments describing how different goals can be reached. UML Use Case descriptions may be seen as a complementary and are equally worded in a mostly unconstrained prose.

TimBox is a quick prototyping tool to incrementally express goals of users and system manipulation. The degree of depth of the model showing these, depends on the system architect's needs at the different stages of his work.

Due to their decreasing abstraction level, task models,enable to state immediately the goals of a user (upper layer) and to describe the segmentation of work (interface manipulation) in the lower layers.

They hereby act as a development guide, where the functions of a system and the interaction of a user with it are expressed in a way allowing an accurate choice of the widgets that will appear on the interface, task after task.

The example below shows the functional specifications of the temperature controlling system of a building as a task model as well as the picture of two interfaces designed after it (for the sake of readability, only the task tree is depicted).