Clarity on the Safety Ruling

I had no idea you could challenge a call like that... At the time, I felt like that was a ticky-tack holding call but I'll live with it. What I don't understand is the ability challenge a penalty like that.

Contact first initiated outside the endzone, so how do we determine "where" the hold occurred? The whole play just seemed super odd to me.

Can you challenge the spot on a PI? can you challenge the spot on a hold? Can you challenge the spot on a fair catch?

I was always under the assumption that judgement calls couldn't be challenged. Help me to understand...