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To answer this, I'd first like to give a short explanation of what a BIP is. BIP stands for "Bitcoin Improvement Proposals" and as the name suggests, community members suggest an improvement for the Bitcoin protocol in an official and structured manner. Every BIP goes through the some phases while it's being decided if it'll be implemented or not, which usually (but not always) look as the following,


So, BIP 174 is going to allow users to perform offline transactions through a protocol that would allow them to sign transactions without having to upload them to the network immediately.
It's also called as "Partially Signed Bitcoin Transaction", PSBT for short.
From the information I've gathered I understand it works almost like a check (cheque) we use for fiat transactions, where we sign in a paper that we'll offer 1000€, give it to a person and then they go to a bank to cash out from our account. In Bitcoin's case we sign a transaction offline, give it to a person, they may even sign it and give it to another person, until finally someone goes online and cash out the BTC from the initial signer.

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Serious explanation ends here, bellow is some extra info I've found for which I'm note 100%.
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The bellow slides 10 and 11 show an example. If my knowledge in Japanese from years of otaku activities are to be trusted,
https://speakerdeck.com/azuchi/bip-174-partially-signed-bitcoin-transaction-format?slide=10
Alice initiates a PSBT transaction and sends it to Bob. He then signs it and sends it to Carol who signs it and sends it to Bob who finnaly recevies the money from Alice. Alice initial transaction was supposed to be for Bob, but since he signed from Carol and then Carol for Bob, Bob is the one who receives the Bitcoins.