Milestones in the context of the Success Lifecycle represent something that is strategic to the building of an organisation and almost all of the time at the Roadmap level. In other words it is likely that for each Roadmap deliverable that there maybe a one to one mapping to a milestone. However there can be more but definitely not equal to the number of Workstream deliverables as that would be too detailed. A milestone typically is something that would need to be communicated to key stakeholders who have an invested interest in the organisation.
A strategic deliverable milestone is a significant event or achievement that marks the completion of a major deliverable in a project. Strategic deliverables are those that are essential to the success of the project and that contribute directly to the achievement of its goals. Milestones are important because they provide a way to measure progress and to identify any potential problems early on. They also help to keep the project on track and to ensure that it is completed on time and within budget.