Original: If available, clarinets in A should be used instead of clarinets in B-flat; the A clarinets would be written in G major, while the B-flats would be written in F-sharp or G-flat major.
Simplified: When writing in E major, clarinets in A should be used instead of clarinets in B-flat to make it easier to play. This is because clarinets are transposing instruments.