Hi @Anonymous -
It looks like you are potentially asking two different questions, one in the subject line and one in the body of your post. I think I can answer both pretty easily.
1. A fixed constraint assumes that the surface, edge or vertex you select is fully fixed from movement. To learn more about this, check out our help here - http://help.autodesk.com/view/fusion360/ENU/?guid=GUID-49E5B765-9042-47DD-BC69-E1D22FD07667
2. Assuming your setup has at least one fixed constraint and one load, the reaction forces will be calculated as part of the stress calculations of the generative design process. There is no reason to apply reaction forces to your setup in this scenario to stabilize the system.
Simply put, you should apply constraints where your model will be restrained from motion and loads where the loads will go into the part.
I hope this helps.
Thanks,
Mike Smell - Product Manager - Fusion 360 Generative Design
