Assemblies, Parts, Folders, Reference Geometry, etc. can all be freely moved or sorted in the Feature Tree in an assembly, by dragging and dropping.
One specific condition I believe may be a bug is as shown in the attached video. As you can see, I have an Axis ("Axis1") just below Origin. I want to move the folder "Ref." from the bottom, to just below Axis1. I'm not allowed. I have to play a game of cat and mouse and move it under the topmost feature, then move the topmost feature below the Ref. folder.
The end result is the same, so SolidWorks mustn't care about what I've done - only that some arbitrary rule exists that you're not allowed to directly rearrange something below a piece of reference geometry. It makes no sense.