I found this while looking through the API:
And I was going to check what it was but NOBODY is talking about it
Please help
Thanks 
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Given that its only reference is a Roblox Security property (you can’t ever access it yourself), I’m assuming it’s an internal way to manage or reconcile the position/velocity between client and server of parts.
It’s nothing you can use so I wouldn’t worry about it.
If you look at the Roblox release that added it (635), you’ll see under its release notes that one change
Fixes bug where deleting a part under the player character may cause them to float in place for other players.
It’s likely related to that.
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ReplicationPV is an internal mechanism for managing part replication and is not something developers can interact with or utilize in their scripts… Its simply an internal Roblox engine type that handles syncing a parts position and velocity between server and client (as @ethamcker said)
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