If the flag_values attribute is present then the flag_meanings attribute must be specified.
The type of the flag_meanings attribute is a string whose value is a blank separated list of words or phrases, each consisting of characters from the alphanumeric set and the following five: '_', '-', '.', '+', '@'.
The number of flag_values attribute values must equal the number of words or phrases appearing in the flag_meanings string.
dim1: [dim2: [dim3: ...]] method [where type1 [over type2]] [within|over days|years] [(comment)]where brackets indicate optional words. The valid values for dim1 [dim2 [dim3 ...] ] are the names of dimensions of the data variable, names of scalar coordinate variables of the data variable, valid standard names, or the word area. The valid values of method are contained in Appendix E. The valid values for type1 are the name of a string-valued auxiliary or scalar coordinate variable with a standard_name of area_type, or any string value allowed for a variable of standard_name of area_type. If type2 is a string-valued auxiliary coordinate variable, it is not allowed to have a leading dimension (the number of strings) of more than one. When the method refers to a climatological time axis, the suffixes for within and over may be appended.
([interval: value unit [interval: ...] comment:] remainder)The remainder text is not standardized. If no interval clauses are present, the entire comment is therefore not standardized. There may be zero interval clauses, one interval clause, or exactly as many interval clauses as there are dims to which the method applies. The value must be a valid number and the unit a string that is recognizable by the udunits package.