"100/(110) VAC" indicates that the coil has 3 ratings.

3 ratings
100 VAC at 50 Hz
100 VAC at 60 Hz
110 VAC at 60 Hz

If the specification has "100/110 VAC," the coil has 4 ratings, i.e., it also has "110 VAC at 50 Hz" in addition to the above. The MY and LY General-purpose Relays are examples of Relays with 4 ratings.