Someone in a Discord group I'm in mentioned "Top Up" games as being even worse than Diablo Immortal and other Gacha games in their microtransactions, but when pressed he couldn't explain the term "Top Up" beyond that it was even worse than Diablo Immortal. I tried googling for it, but the only article I could find that wasn't from a site that was trying to sell them said as the very first line "you probably already know what a top up is, so I'm not going to explain it" which didn't help me at all.
So what does it mean for a game to be a "top up" game?