Universality classes identify systems with the same long-distance renormalization-group fixed point. Spatial dimension, order parameter symmetry and number of components, and the range of interactions help determine the equilibrium class. Microscopic lattice details and many local interactions become irrelevant operators under coarse-graining. Universal quantities include critical exponents, scaling functions after fixing metric normalizations, and suitable amplitude ratios. The critical temperature, bare couplings, absolute correlation-length and susceptibility amplitudes, analytic free-energy density backgrounds, and microscopic length scales are generally not universal. Dynamic universality can additionally depend on conservation laws, so it should not be inferred solely from a static scalar potential.
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