DietersMistakesIn2021/Solution2

Mistake 2  

Answer: the top is relatively small and it's not clear how to use the remaining aji. The left side is the big/urgent area to play.

Good exchange for white  

After this , black has maybe 20 points of territory at the top. 20 points would be big for an endgame move. But at this stage of the game, and together are worth more than that. Black would do better to answer at a.

Other options  

For me, at c is not intuitive! (This is a problem with my intuition. It's a fine move.) I was tempted to play instead at either d or e in this diagram. My first choice would be d, thinking that I want to restrict Black's moyo at the bottom.

KataGo tells me that the difference between all these options is fairly subtle. Even the worst of these options, at a, only loses about 2 points compared with c. All of b, d and e are about equal to each other, maybe 1 point worse than c.

Followup to d  

If here, KataGo recommends -. I guess this is a form of aji-keshi: the - exchange loses the chance to invade around a.