This is a projection manifold for tri-planar mapping. This techniques is also sometimes called "blended cube". It is often used to texture large environment without having to worry about UVs.
PxrRoundCube computes texture coordinates (uv) based on an object's surface position and orientation and can handle up to 6 sets of coordinates. It also outputs blending weights to be used by PxrMultiTexture.
PxrRoundCube only defines texture mapping coordinates and MUST be connected to a PxrMultiTexture pattern. To do so, connect PxrRoundCube.resultMulti to PxrMultiTexture.manifoldMulti. See maya example below.
NOTE: In RenderMan 20, PxrRoundCube used to handle texturing on its own and you will need to modify your shading networks when importing old scenes.