The harmonization tutorials included come from pages 39 and 40 in your text. The whole purpose is to give you some rationale for chord selection as well as examples of accompaniment styles.
p.39, item 3.b.
F# is the 3rd of the tonic or the root of the iii chord
A is the root of the V, the 3rd of the iii, the 5th of the tonic
B is the 5th of the ii or the 3rd of the IV
A is a member of the I,iii or V - G is a passing tone
F# & A function as the 3rd & 5th of tonic, the root & 3rd of iii
B is the 5th of ii, the 3rd of IV
F# & D function as the 3rd and root of the tonic triad
E is root of ii or 5th of V - at final cadence you might use both!
