For a subshift (X, σ X ) and a subadditive sequence F = {log fn} ∞ n=1 on X, we study equivalent conditions for the existence of h ∈ C(X) such that limn→∞(1/n) log fndµ = hdµ for every invariant measure µ on X. For this purpose, we first we study necessary and sufficient conditions for F to be an asymptotically additive sequence in terms of certain properties for periodic points. For a factor map π : X → Y , where (X, σ X ) is an irreducible shift of finite type and (Y, σ Y ) is a subshift, applying our results and the results obtained by Cuneo [9] on asymptotically additive sequences, we study the existence of h with regard to a subadditive sequence associated to a relative pressure function. This leads to a characterization of the existence of a certain type of continuous compensation function for a factor map between subshifts. As an application, we study the projection πµ of an invariant weak Gibbs measure µ for a continuous function on an irreducible shift of finite type.