= Solution
Microscopically different systems can have the same <critical exponents> and appropriately normalized scaling functions. This universality means that they belong to one <universality class> when coarse-graining brings them to the same <renormalization-group fixed point>. Spatial dimension, <order parameter> symmetry and the range of the interactions affect that classification; changing a lattice spacing or a microscopic coupling generally changes nonuniversal amplitudes and the critical temperature instead.
In the scalar short-range <Landau-Ginzburg theory>, a positive quartic term and one thermal tuning produce ordinary Ising-type criticality. A generic sextic term does not change that fixed point, but tuning the quartic coefficient to zero exposes a different <tricritical point> with different exponents. The <mean-field approximation> itself neglects fluctuations. Its numerical exponents are reliable only when the <Ginzburg criterion> makes those fluctuations subleading; the ordinary scalar <upper critical dimension> is four, while the tricritical one is three. At an upper critical dimension logarithmic corrections can remain.
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