Being used as inputs in the same transaction essentially demonstrates they are all controlled by the same owner.
A large number probably represents a mining pool or exchange, a small number is probably just an individual.
Please Note: This does not represent all addresses with the same owner, only those addresses used together as inputs to one or more transactions.
A collection of 'seen with' addresses is referred to as a 'folio'.
The address with the highest balance in that folio is considered the parent of that folio.
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