What’s a possible one-word replacement for “applicable in every situation”?
Solution 1:
Universal
adapted or adjustable to meet varied requirements (as of use, shape, or size)
a universal gear cutter
universal remote control
Solution 2:
General, especially in mathematics:
involving, relating to, or applicable to every member of a class, kind, or group
the general equation of a straight line
One option you have is to say what the equation is rather than what it isn’t, such as: “This equation applies only to a special case.”