The XGen Interactive Groom Splines Attributes can be used to fine tune the rendering of XGen splines (hair) with V-Ray. They allow you to control the hair tessellation during render times. To apply it select the descriptionShape node and go to Attributes > VRay menu in the Attribute Editor.

UI Path: ||Select descriptionShape|| > Attribute Editor >  Attributes menu > VRay > Hair Geometry

Dynamic Hair Tessellation – When enabled, V-Ray dynamically changes the subdivision of the hair based on the distance to the camera. This means that strands that are closer to the camera are subdivided more times than strands that are far away. This allows for a better memory utilization.

Segment length (px) – When the Dynamic Hair Tessellation is enabled this parameter controls the size of the segments generated by the subdivision. A value of 4 means that a segment is going to be 4 pixels long in the final image. This way segments that are far away from the camera are going to be larger in real world units.

Min pixel width – Sets the minimum pixel width of each hair strain to help ensure samples are taken consistently between pixels. A value of 0 means no minimum width is applied, but increasing it helps make sure each strain is consistently rendered. This additional calculation increases render times, but smooths the hair width. Note: Values are limited to 2. Larger values (closer to 2) can significantly increase render times.

Max tessellation – Allows the user to limit the number of segments when hair is dynamically tessellated. When set to 0 (default), V-Ray doesn't limit the number of segments. When set to a value greater than 0, each hair strand is dynamically tessellated but contains no more than the specified number of segments. This can be helpful in reducing memory usage.