The ability to "Grandfather"
legislation,
regulation, or rules of any nature is a
powerful tool.
When a guideline of any nature is created or modified there is always the question about what to do with the instances under
purview that occured
prior to its implementation.
One way a rule is "grandfathered" is when it retroactively applies to any instance that occurred in the past. For example, if a rule came out making marijuana legal, there would be a massive battle on how far back and to what extent the new law would apply to people convicted under the previous set of laws. That's why most grandfather clauses have very tight restrictions on how they may be applied.
A "grandfathered" law may also refer to exemption of those violating the rule prior to its inception. For example, when Washington, D.C. raised its drinking age from 18 to 21, people between those ages, who could drink under the old law, were allowed to retain the right to legally consume alcohol under a grandfather clause.