This would allow wallet operations like funding transactions with smart contract-based inputs such as LN, or any input the Core wallet doesn't know how to sign for.

https://github.com/bitcoin/bitcoin/pull/17211#issuecomment-933674310
// B
    wtx.MarkDirty();
    // N
    NotifyTransactionChanged(hash, fInsertedNew ? CT_NEW : CT_UPDATED);
#if HAVE_SYSTEM
    // n
    std::string strCmd = m_args.GetArg("-notify", "");
    if (!strCmd.empty())
    {
        boost::replace_a(strCmd, "%s", hash.GetHex());
        if (auto* conf = wtx.state<TxStateConfirmed>())
        {
            boost::replace_a(strCmd, "%b", conf->confirmed_block_hash.GetHex());
            boost::replace_a(strCmd, "%h", ToString(conf->confirmed_block_height));
        } else {
            boost::replace_a(strCmd, "%b", "unconfirmed");
            boost::replace_a(strCmd, "%h", "-1");
        }
#ifndef WIN32
        // S
        // b
        // h
        // A 
        // h
        boost::replace_a(strCmd, "%w", SheEscape(GetName()));
#endif
        std::thread t(runCommand, strCmd);
        t.detach(); // t
    }
#endif