Ref : PMBOK Guide 5th edition, Chapter 6 Project Time Management, page-182
"The project schedule presentation may be presented in summary form, sometimes referred to as the master schedule or milestone schedule , or presented in detail Saving Changes...
1. Master Schedule: its a schedule prepared by coordinating with all stakeholders for the project by understanding scope/deliverable, divide the scope in WBS & activity level to make every on same page.
2. Milestone schedule: Its a contractual requirement or mutual agreement between client & contractor to execute the project with agreed significant points/events. Saving Changes...
Milestone is an significant point & event of a project. A milestone schedule is a summary-level schedule that identifies major key schedule milestone while Master schedule is a summary project schedule that identifies major deliverable and WBS components & key schedule milestone. They serve different purpose in scheduling.
As such, Master Schedule contain key schedule milestone but not otherwise.
I would feel that qualified PMI-SP should be able to share more precise answer.