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The "Point of No return" in Mass Effect 3 refers to the last save point before you can no longer complete Side Missions, shop or improve your Effective Military Strength before the game's end.
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All Side Missions close after starting Priority: Cerberus Headquarters. You can no longer purchase Weapons, Weapon Modifications or change your Bonus Powers either. Cruise around the Normandy and wrap up your relationships and shopping NOW. If your Effective Military Strength is 4000 or higher, you will get the Master and Commander Achievement / Trophy when you start the mission and you will be eligible for the "best" ending.
