At issue in Moore v. United States is the question of whether the federal government can tax certain types of “unrealized” gains.

Unrealized gains come from assets people own, but from which they haven’t directly recouped the value, like stocks or bonds that have increased in value since purchased.

Large portions of the U.S. tax code require that income be “realized” before it can be taxed.

Critics say it’s an inherently wishy-washy concept that courts have just been ignoring for years due to administrative impracticalities.

Even if the court limits the scope of its decision to the specific tax referenced in the case, known as the mandatory repatriation tax, a ruling in favor of the plaintiffs could cost $340 billion over the next decade, according to the Justice Department.

“If [the court makes] a specific realization requirement, then it could have an impact on many other provisions of the Internal Revenue Code,” Lawrence Hill, a partner at the Steptoe & Johnson law firm, told The Hill

Kyle Pomerleau, a senior fellow at the American Enterprise Institute who filed an amicus brief supporting the government, said a broad ruling in favor of the Moores could lead others to file a deluge of lawsuits challenging those other portions of the tax code.

“A cloud is going to be cast over the U.S. economy, as there’s all this uncertainty about what the tax code is going to look like in five, 10, 15 years from now.”

The Hill Tobias Burns and Zach Schonfeld has more here.