what is 'selection alpha' in scripting

Hello,

I am learning after effects expressions. The tutorial am following, there the tutor said the student and the teacher need to have same settings so taht we are on the same page throughout the class.

I notice there is something called 'Selection alpha'. My tutor's is 0.37 and mine is only 0.3

am curious what is it actually. I googled nothing relatable with scripting. but general discussion about alpha channel of layers or other stuff.

what is 'selection alpha' in scripting? should I change the  number?