What must be true for an expression to be a difference of two squares

I think it must have a power like squared or cubed also I think it must have a subtraction sign ...

Some examples of difference of two squares:

9x^2 - 1

Which would be = (3x+1) (3x-1)

4n^2 - 49

Which would be = (2n-7) (2n+7)

249r^2 - 150

Which would be = 6 (7r+5) (7r-5)

150k^2 - 216

Which would be = 6 (5k+6) (5k-6)

Notice that each bracket for each answer, one bracket has a + sign and another has a - sign.