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Milestone (M) builds occur throughout the development cycle when a significant set of features or bugs have been implemented. The build will be stable enough for testing those new changes but may have introduced new bugs or instabilities to other parts of the code. Milestones give the community an opportunity to provide feedback on a feature while there is still time to change it during that dev cycle. Milestone builds should never be put into production.
Pre Release builds occur at the end the development cycle when a significant set of features or bugs have been implemented. The build is stable enough for testing those new changes but the full testing that goes into a Release Candidate has not been completed. Pre Release builds are only used when there is a compelling reason to get the release out to the community for feedback prior to the code cutoff period prior to RC testing. Pre Release builds should never be put into production but can be used for POCs and prototypes.
Release candidate (RC) builds are much more stable then milestones and come at the end of the development cycle. There can be as many RC builds as needed until the release is considered stable. RC builds are ready for full testing and give the community time to find critical bugs or usability issues before the release is marked stable. RC builds should not be put into production but can be used for acceptance testing or prototyping.
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