After reading this question on how it is calculated, I was wondering what it means.
What does the 'Estimated BTC Transacted' number on blockchain.org refer to? What does it mean?
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The total estimated value of transactions on the Bitcoin blockchain (does not include coins returned to sender as change).
The salient point in the above description is "does not include coins returned to sender as change". Since there is no reliable way to definitively know what outputs are change outputs, there is no way to be 100% certain of the actual volume of BTC transacted. Blockchain.info makes educated guesses about the likely change outputs, which it deducts from the total transacted to obtain the estimated BTC transacted value.
For instance, if you look at this transaction, you can see a single input of 0.565975 BTC, with two outputs, one for 0.00145 BTC and one for 0.56 BTC. While it seems reasonable to assume the transaction "spent" 0.56 BTC, with "change" of 0.00145 BTC, it may very well be the other way around. In fact, blockchain.info believes the 0.00145 BTC is the actual transaction amount, with 0.56 BTC being the change amount.