What's the difference between the base and reserves set?

[johnny]

I am buying the minis for someone else for a gift.  I know they have the starter set and one booster.  Do I need to continue to buy the base set or “reserves”?  Something else? Thank you.

[dezrtfish]

The Base set is the first set, reserves in one of the later sets.

1st release - Base set
2nd release - Set II (sometimes called Eastern Front)
3rd release - Contested Skies
4th release - D-day
5th release - Reserves

Each of the releases has different units, but they all play well together ;)