See release notes at https://github.com/JoinMarket-Org/joinmarket-clientserver/blob/master/docs/release-notes/release-notes-0.5.4.md
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This release includes basic coin control (used on the command line with the wallet-tool.py method 'freeze', see the help, or in the Qt GUI use the new 'Coins' tab), which allows you to freeze individual utxos so that they will not be spent in any transaction, and a few minor doc updates and bugfixes.