What different criteria would you choose to classify people that you meet often?

Consider the various options we have when we choose a fuel for cooking our food. What are the criteria you would consider when trying to categorise something as a good fuel ?

What universal set would you propose for each of the following : The set of squares.

What universal set would you propose for each of the following : The set of rectangles.

What universal set(s) would you propose for each of the following : The set of isosceles triangles.

What universal set would you propose for each of the following : The set of circles with unit radius.

What universal set(s) would you propose for each of the following : The set of right triangles.