Only the first two make sense and are valid in Dynare. @stepan-a linked to a proper example file. Note that that function is used for a stochastic model where the exo argument is not used. What that input does is providing the value of the exogenous variables at which the steady state is computed. You need this in perfect foresight models if you have permanent shocks and therefore different steady state values at the beginning and end. See e.g. Permanent Shock in deterministic model