A drop ball is performed whenever the game is stopped for a reason other than the other 7 prescribed restarts. Think of it as a catch-all.
In practice this is most often a no-fault restart. A dangerous situation for really young kids (where no single player is at fault) or interference from off the field, maybe a dog runs in and takes off with the ball.
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Usually an injury to player whereby the referee stops play but doesn't give a free kick.
Any stoppage in play that is not a free or penalty kick. So if a player gets hurt or a ref needs to remove an object from the field, etc . . it would be a drop ball.
As the game is in progress the referee would restart play following
foul play
Injury stop
Penalty
Goal scored (kick off from centre spot)
Free Kick