Prashant, think of the milestone as a gate. You do lots of work and tasks to get there, so once you arrive, you go through that gate. Don't stand in the gate and wait. Just pass through. In real life and for the exam, the Milestone will be the intermediate goals throughout the project. It might consist of an approval presentation to your boss or acquiring a key resource to start production.
The milestone list in an Output from process 6.2 Define Activities (page 149 in the PMBOK)
and you are right the key milestones are also defined in the Project Charter and are an
Input to "6.1 Plan Schedule Management" and the Schedule Management Plan is an Input this process.
In general a milestone is a significant point or event in a project, program or portfolio.
A "milestone list "is a list identifying all project milestones and normally indicates whether the milestone is mandatory or optional.
So to answer your question already here you can assume a milestone is similar to a regular schedule activity with duration zero which will be defined in the process Define Activties.
Further more in process 6.2 Define Activities you are now the first time able to write done all milestones (duration zero) and activities in lists .
Why the first time ?
Your Scope Basline is ready ("What Plan") and your Schedule Management Plan is ready ("How to plan") and now
you can sit down the first time and create the "raw data" for the process "6.3 Sequence Activities".
Well, I'm listening to PrepCast and It is unclear, WHY milestone list suddenly appears after Define Activity process? We didn't even mention it in PrepCast. Isn't milestone list closer to Sequencing? Or even to Develop Schedule? Milestones may come from Project Charter, Create WBS Process, EEF, OPA... AND from Define Activities for sure, but why this list is an output of Define Activity?!
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