Task Descriptor: Define the Architecture
Define a candidate architecture for the system based on experience gained from similar systems or in similar problem domains. Define the architectural patterns, key mechanisms, and -- where applicable -- modeling conventions for the system.
Based on Method Task:  Define the Architecture
Relationships
Steps
Identify architectural constraints

List any constraints on the solution (or possibly opportunities) that arise from the existing environment or organisation. 

Survey, assess and select from available assets

Establish the availability of suitable candidates for re-use from the existing software estate.

Identify analysis mechanisms

Catalogue the architecture mechanisms needed to support the requirements (see Architecture Mechanism). These will be expressed as Analysis Mechanisms at this point in the process, due to the relatively small amount of detail required against each mechanism.

Map each mechanism to one or more use cases. This will how the mechanisms are used to support the requirements and assist in the prioritisation of the development of the mechanisms.

Capture architectural decisions

Keep a log of important decisions for future reference.

Properties
Multiple Occurrences
Event-Driven
Ongoing
Optional
Planned
Repeatable