Project milestones

After you define the objectives, budget, and schedule for each phase and
prioritize risks, then you can identify project milestones and describe exit
criteria for each one. A milestone marks an important point in time at
which you can assess a particular artifact, synchronize activities, or deliver
a product. Milestones that mark the end of a phase (referred to as major
milestones, i.e. customer acceptance of product) also serve as important decision-making moments. At these
milestones, stakeholders assess the results of the preceding phase and
give formal approval to proceed with the next phase of the project. In
order to set clear expectations and ensure that each phase is assessed
objectively, you must specify exit criteria in advance for each milestone.
If you have done a thorough risk analysis, defining milestone exit criteria
for the Inception and Elaboration phases will be easy. If you find yourself
struggling to refine the default criteria described in RUP, you should revisit
the Prioritize Risks activity.
Exit criteria for the Construction and Transition phases will be more
difficult to define at this stage. Usually, the default criteria are a good
place to start; you will refine these criteria sometime during Inception or
Elaboration. Formulate milestone exit criteria as closed yes/no questions
because you will use them to determine whether the project may proceed
to the next phase.
For example, we have milestone "Beta release", and  exit criteria could be- is the product sufficiently complete and of sufficient quality to start production acceptance testing? Or, have the end-user and support
organization been prepared for deployment?Are budget/schedule variations acceptable to the
stakeholders?
Be sure not to include the state of a particular artifact or activity in these
criteria. Simply producing an artifact or performing an activity is not in
itself valuable to the project. Rather, it is what you achieve by producing
the artifact or performing the activity that is valuable.
source: The Rational Edge -- August 2003

Example of poor milestone exit criteria- have the test cases been executed?

Better criteria- do stakeholders find the quality of the product acceptable?