Minecraft wiki entry for Target_selectors, associated text: @a[nbt={OnGround:true}] — Select all players on the ground.
@e[type=sheep,nbt={Color:0b}] — Select all sheep that are dyed white.
@e[type=item,nbt={Item:{id:"minecraft:slime_ball"}}] — Selects all slime ball item entities.
@e[nbt={Tags:[a,b]}] is the same as @e[tag=a,tag=b]. The latter is simpler and reduces CPU load.