The recently released software release BTC Core 0.16.0 contributed to the reduction of commissions and facilitating blocking, significantly simplifying the creation of SegWit-wallets and increasing the number of SegWit-transactions. The next version of the software, the release of which is scheduled for 2019, will change one of the core mechanisms of bitcoin-transactions - the algorithm of selection of coins.To understand the principle of its work, you can imagine the bitcoin-transaction as a cash payment process, where you pay a $ 10 bounty for a thing worth $ 7 and get $ 3 change. The bitcoin, which stores, for example, 2 bitcoins, contains not two separate coins, but also the so-called "unspent transaction yields" (UTXO) - "balances" of previous sent or received transactions that can represent a sum of 0.1 BTC, 0.3 BTC, 0.1 BTC, 0.5 BTC, 0.2 BTC, 0.4 BTC, 0.3 BTC and 0.1 BTC or any other combination of similar (and even smaller) components. Deepening further, these records are not contained in any particular address or purse, but are scattered among a multitude of transactions and blocks. The wallet calculates the user's balance by scanning the blockage and finding all UTXOs belonging to that user. That is, each bitcoin-transaction consists of various smaller amounts of bitcoin, and if you want to send 0.2 BTC, the coin selection algorithm in Bitcoin Core can send a data segment storing 0.3 BTC to the "input of the transaction" (input - a record from which addresses of bitcoins are obtained) and create two outputs (output - to which address the bitcoins are sent): 0.2 BTC will be sent to the recipient, and 0.1 BTC will return to the purse as a "change".
Read more hear: https://decenter.org/coins/421-changing-bitcoin-rus
