Novice API question about $uuid=null parameters

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What does this mean? On the GET api calls with this in the parameter column I keep getting 200 back, but empty braces. The other calls that don’t have this work fine. I’m specifically using it with ZT network api on OPNsense (/api/zerotier/network/info), but I imagine this has a wider reach than just that. I’ve tried using $uuid=null, uuid=null, $uuid=(acutal uuid for zt net here), and uuid=(acutal uuid for zt net here). Not to mention I don’t know what the $ is all about either. Yes, I know JUST enough to get myself into trouble so if you would like to tell me off right now I would understand. I just haven’t been able to find anything coherent on the subject so I ask as a last resort. Thank you for your patience.