Pair_coeff command problem

pair_coeff * * library.meam Fe Cu FeCu.meam Fe Fe Cu
pair_coeff * * library.meam Fe Cu FeCu.meam Fe Cu Fe
what is the difference between the above two command in lammps?

atom type 1 and 2 are Fe and atom type 3 is Cu in the first line
atom type 1 and 3 are Fe and atom type 2 is Cu in the second line

That is probably impossible to say just by looking at an input. Note also that posting a different question within the same topic is not appropriate.
Best,
Simon