The API Help says that 'Most parameters are dimensions' which means that some other types may be also returned. But from my practice I have never seen the case when it is used for different purposes rather than dimensions.
There is no separate interface for individual equations in SolidWorks. You can traverse and change the equation through the IEquationMgr interface.
Please check the following example: http://help.solidworks.com/2013/English/api/sldworksapi/get_equation_values_example_vb.htm
Artem Taturevych, Application Engineer at Intercad (Australia)