The implementation plan establishes the schedule and needed resources. It defines implementation details including programming languages, platforms, programming environments, debuggers, and many more.
The implementation plan could be considered as part of the design, which is the position taken here, or it could be considered as the first accomplishment in the implementation phase. One of the goals of the design phase is to establish a plan to complete the system. Thus it is very natural to include the implementation plan. Also, the trade-offs between alternative architectures can be influenced by differences in their implementation plans.