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The symbols aux, const, data, levels and rates represent different types of variables.

auxt Auxiliary.  These are computed (see equation 3) from Levels, Constants, Data, and other Auxiliaries.  Auxiliary variables have no memory, and their current values are independent of the values of variables at previous times.
const Constants.  These do not change with time.
datat Data (also called exogenous).  These have values that change over time but are independent of anything that happens to other variables.
levelst Levels (also called accumulations, stocks and states).  These change only over time and the values they take on at any time depend on the value they (and other variables) took on at previous times.  Equation 1 shows how the Levels integrate or "accumulate" based on the values themselves and other variables in the system.  The Level variables ultimately determine the dynamic behavior of a system.
ratest Rates (also called flows).  These are the variables that directly change the Levels.  Rates are essentially the same as Auxiliaries and differ only in the way they are used in a model.

In the Vensim modeling language, Rates are implicitly determined based on Auxiliaries and other variables, and are not broken out as a separate variable type.  Put another way, an Auxiliary that is used to change a Level can also be thought of as a Rate.

In addition to Auxiliaries, Constants, Data and Levels the Vensim modeling language contains a number of additional variable types that make analysis easier and more powerful.