CFSN0371	sea_ice_volume		"X_volume" means the volume occupied by X within the grid cell.	M
CFSN0364	sea_ice_extent		The term sea_ice_extent means the total area of all grid cells in which the sea ice area fraction equals or exceeds a threshold, often chosen to be 15 per cent. The threshold must be specified by supplying a coordinate variable or scalar coordinate variable with the standard name of sea_ice_area_fraction.  The horizontal domain over which sea ice extent is calculated is described by the associated coordinate variables and coordinate bounds or by a coordinate variable or scalar coordinate variable with the standard name of "region" supplied according to section 6.1.1 of the CF conventions.	M