exactly! it all depends, because the dynamic markings are written by the composer.. so it's all about what the sign meant to the composer. and we can't use a dictionary to decipher that.. i seem to recollect someone saying that for example for Schubert dim. usually meant decrescendo+ritardando.. but i couldn't give you a source for that, i no longer remember. then i guess you've heard about that sets of opposing hairpins were used to mark rubato in some traditions..
