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A soft fork is backward-compatible, a hard fork isn't.
A hardfork is a change to the bitcoin protocol that makes previously invalid blocks/transactions valid, and therefore requires all users to upgrade.
https://en.bitcoin.it/wiki/Hardfork
A softfork is a change to the bitcoin protocol wherein only previously valid blocks/transactions are made invalid. Since old nodes will recognize the new blocks as valid, a softfork is backward-compatible. When a majority of miners upgrade to enforce new rules, it is called a miner-activated softfork (MASF). When full nodes coordinate to enforce new rules, without support from miners, it is called a user-activated softfork (UASF).
Alright, so what do we call the state of having 2 chain tips where the UASF side is a persistent minority chain? Because that scenario isn't covered in this terminology. Bitcoin Unlimited retroactively decided to do the equivalent of lot=true after their MASF failed, which is the closest comparison we have here, except now it is coded in. (iirc we called the BU decision a hard fork, but whatevs)
So, if the majority of all economic actors is going to enforce the UASF, to which we have no indication with just a few weeks left, fine. But what will we end up with if that too[1] fails? It's not a softfork if I need to run a second client to see the minority chaintip.
assuming that with 0.77% signal for bit 4, the MASF component will fail. ↩
I'd call it a permanent chain-split caused by a soft fork with minority hashrate support. The Bitcoin rules would be the hard fork to RDTS, but RDTS changed the rules in a compatible manner and Bitcoin didn't, so calling it a hard fork would be weird.
Hard/soft fork refers to the type of rule change. Chain split describes the outcome.
I wrote as much a long while about it, so that distinction seems to be quite old: https://bitcoin.stackexchange.com/a/30821
How should we call an irreconcilable difference between two clients where one version rejects blocks made by the other, regardless of adoption? Asking because I'd call that a hard fork, but maybe I'm using the wrong word.