What Can I use an A5 notebook for?

Ok so I have three different sizes of notebooks: A4 which I am using for my lecture notes and a B5 which i want to use for exam papers and tutorial questions and answers but I also have an A5 notebook that I want to use for university but idk what I should use it for? I was thinking maybe summaries or formulas? But I want to make sure I really do use it for something useful.

I do not want a daily planner because I already have one so what are your suggestions?