Garageband throws security alert with Spitfire Labs plug in

[Doctor Blue]

A few days ago I downloaded and installed Spitfire's app and one plug-in ("soft piano") on my new MacBook Pro. When I opened Garageband and tried to load the Labs plug-in, I got a warning that said something to the effect, "in order to use this Audio Unit, lower your security settings in Garageband. Do you wish to proceed?" Does anyone know what that is? (I successfully downloaded a different company's plug in without issue). I spent way to many days searching for an answer about it without any luck and so ended up just allowing it, and the Spitfire interface loaded just fine. But I could not find any information about this alert or why it was happening.

Any insights would be most helpful. Spitfire has not been useful in this regard. TIA.