The name used in an expression can be either a fully-qualified variable name, a dataset-qualified variable name, or a primary attribute. (Currently the attribute name can also be an un-qualified attribute for an existing attribute of the dataset, but support for this may be dropped in future versions of Opus. So we recommend always qualifying the attribute name with the name of the dataset in an expression.)
The variable names can include arguments (Section 6.3.4), so that is_near_highway_if_threshold_is_2 matches the variable definition for is_near_SSS_if_threshold_is_DDD.