What are value field and definition field

What are value field and definition field

For example: function y = 2 ^ x ∵ Y > 0 [no matter how x takes value, y cannot be equal to 0 or less than 0] ∵ the value range of this function is y ∈ (0, + ∞), and the independent variable x can take any value, so the definite value of this function