When displaying fog volumes, the viewport can sample the underlying textures using either fast trilinear interpolation or slower (but smoother) b-spline interpolation. This enumeration contains the various sampling modes supported by the viewport.
The equivalent setting in the viewport display options can be found in the Geometry tab and is labelled Volume Filtering.
The viewport will never use higher-order volume interpolation. This is the fastest to draw, but low resolution volumes may appear blocky in the viewport.
The viewport will use the higher-order interpolation when not interacting with the viewport. This mode falls back to linear interpolation while interacting with the viewport to speed up draws.
The viewport will always use higher-order volume interpolation. This is the slowest, highest quality option.