By definition Total slack is how much we can delay the START of an activity. But what type of slack do we have for delaying the FINISH of an activity?

I have an activity that has just one successor, and it is linked as

Activity 2 = Activity 1 SS + x days

My schedule says that Activity 1 has total slack of, say, 2 days.

But even if I increase the duration of my activity by more than 2 days, its total slack doesnt change.

So it means it is only affected by the start date of that activity. What type of slack do we have for delaying finish of it?