Battlefield Control is, in essence, being able to manipulate how your opponents are able to attack you.

If you cast a Black Tentacles at opponents, tangling them up so the party beatstick can take them one at a time rather than all at once, that is battlefield control

If you cast Solid Fog, which slows down opponents, allowing you to split them up, that is battlefield control.

If you are a 'Gattling Tripper' who makes all opponents within 30' of you fall down, that is battlefield control.

If you are using Dominates to turn enemies into temporary allies, that is battlefield control.

Basically, anything other than buffs, out of combat utility, and blastomancy/Save or Die effects can generally be considered battlefield control.