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I want to draw a semi-transparent ball and highlight its intersection with a plane.

ℛ = ImplicitRegion[x^2 + y^2 + z^2 <= 1 , {x, y, z}];

r3d = 
  RegionPlot3D[ℛ, 
    PlotRange -> {{-1.5, 1.5}, {-1.5, 1.5}, {-1.6, 1.6}}, 
    PlotPoints -> 30]

inter = 
  Region[RegionIntersection[ℛ, InfinitePlane[{{1, 0, 0}, {0, 1, 0}, {0, 0, 1}}]]]

Now why won't this work?

RegionPlot3D[inter, 
  PlotRange -> {{-1.5, 1.5}, {-1.5, 1.5}, {-1.6, 1.6}},  
  PlotPoints -> 30]

I have a implicit region and a plane, I form the intersection, but I'm unable to plot it.

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On the one hand inter is already a Region and need not be wrapped by RegionPlot3D. RegionPlot3D wants to have a boolean expression as first argument; since an ImplicitRegion essentially consists of such a boolean expression, RegionPlot3D was also overloaded for ImplicitRegions.

On the other hand, is a ImplicitRegion; these are not plotted automatically. But you can turn it into a Region with Region[ℛ] to obtain a preview or apply DiscretizeRegion or DiscretizeBoundaryRegion to compute polyhedral approximations.

As side remark: RegionPlot3D was introduced in version 6.0 while Region was introduced in version 11.1. So RegionPlot3D follows an older mechanic; it was not meant as displaying/plotting function for Regions, even if the name might suggest that.

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