Subject: [mongodb-user] Chunks Migration violates Shard

I have a question related to a chunks migration in a MongoDb Balanced Sharded Cluster.

According to Sharded Cluster Balancer manual page, on reaching some thresholds, balancer starts actual chunks migration between shards to balance the chunks distribution.
I don't understand and I could not find a clear answer - how does this process corellate with a shard key range?
How can a chunk with its own range of data based on a shard key be migrated to another shard that simply cannot hold that data because it has different shard key value boundaries?
I can assume this when a hashed sharding strategy is used - as I got it in this case mongodb operates hashed shard key value boundaries for shards on its own and probably can change them (???).
But I cannot understand this for a ranged sharding strategy.

E.g.,, I have 2 shards with a collection sharded on an X key:
1st holds data for X values from 1 to 1mlrd
2nd holds data for X values from 1mlrd to 2mlrd

How can a chunk from the 1st shard migrate to the 2nd when its data's shard keys doesn't fit into the 2nd shard shard key's value range?
Am I getting smth wrong about all this "chunk migration"?

Please, help. I'm stuck )))

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