In some situations a dimension may have alternative sets of coordinates values. Since there can only be one coordinate variable for the dimension (the variable with the same name as the dimension), any alternative sets of values have to be stored in auxiliary coordinate variables. For such alternative coordinate variables, there are no mandatory attributes, but they may have any of the attributes allowed for coordinate variables.
Example 6.3. Model level numbers
Levels on a vertical axis may be described by both the physical coordinate and the ordinal model level number.
float xwind(sigma,lat);
xwind:coordinates="model_level";
float sigma(sigma); // physical height coordinate
sigma:long_name="sigma";
sigma:positive="down";
int model_level(sigma); // model level number at each height
model_level:long_name="model level number";
model_level:positive="up";