What is the "revenue rule" in common law?

The "revenue rule" -- a firmly embedded principle of common law, traced to an opinion by Lord Mansfield, Holman v. Johnson, 98 Eng. Rep. 1120 (K.B. 1775) -- holds that courts generally will not enforce foreign tax judgments, just as they will not enforce foreign criminal judgments.... The rationale of the revenue rule has been said to be that revenue laws are positive rather than moral law; they directly affect the public order of another country and hence should not be subject to judicial scrutiny by American courts; and for our courts effectively to pass on such laws raises issues of foreign relations which are assigned to and better handled by the legislative and executive branches of government.