bitcoin node: what is the difference between simnet and regtest?
Simnet is only available for btcd (bitcoin node) and lnd (lightning node) afaik, while regtest is also available for bitcoind (bitcoin node), lnd and eclair (lightning nodes). The two modes are similar, according to documenation of LND:


  Simnet is similar to regtest in that you'll be able to instantly mine blocks as needed to test lnd locally


Both also are similar in the fact that the payment addresses are unique, to prevent confusion with the main network.

Where they differ, I think, is that simnet goes a bit further and also applies some network changes to disable things like peer discovery.

But I couldn't find conclusive evidence for this. Since I do a lot of tests with multiple Lightning clients, I tend to use regtest, since that's more broadly supported.