Master Attributes are typically the heavyweight geometric data that is best shared for efficiency reasons, like normals, vertices, etc. Note that these attributes can only be used on the Master object. You can find these listed here.
Instance Attributes allow for variation and more for multiple different instances and are only available to instances, not the master. Things like bxdf assignment, light linking, trace sets, and more can be assigned per-instance. You can find this list here.
RenderMan tessellates geometry using an adaptive technique such that the resulting micropolygons have a length measured in a small number of pixels. This is typically the instance closest to the camera. However, this technique is not useful because with this screen projection based technique, no single tessellation can be reused for an object master instanced at different distances from the camera. Instead, the default tessellation is a world distance based measurement: the micropolygon length is measured directly in the units used in the scene, without projecting to the camera.