Justice Amy Coney Barrett, in a concurrence to Friday's ruling striking down President Joe Biden's student debt forgiveness plan, defended the Supreme Court conservative majority's use of a controversial legal theory and -- deploying analogies concerning babysitters and grocery store owners -- laid out how she believed the doctrine should be used in the future.

The so-called major questions doctrine says that executive branch agencies only have authority to take aggressive unilateral action of significant political or economic importance if Congress explicitly gives it such power.

In the student loan case, the 6-3 conservative majority, including Barrett, concluded that the student loan law in question did not give the secretary of education the power to cancel broad swaths of loans.

Barrett expounded on the utility of the doctrine by spelling out scenarios where, she writes, context is key to interpreting the limits of authority Congress has delegated to an agency.

The grocer hypothetical concerned a store owner who typically keeps 200 apples on hand then instructs a clerk to "go to the orchard" to buy more apples.

"Though this grant of apple-purchasing authority sounds unqualified, a reasonable clerk would know that there are limits," she wrote.

"For example, if the grocer usually keeps 200 apples on hand, the clerk does not have actual authority to buy 1,000 -- the grocer would have spoken more directly if she meant to authorize such an out-of-the-ordinary purchase," Barrett wrote. "A clerk who disregards context and stretches the words to their fullest will not have a job for long."

A second hypothetical centered on a babysitter who took the kids to an amusement park for the weekend, having been given a parent's credit card and told: "Make sure the kids have fun."

"Emboldened, the concurring babysitter takes the kids on a road trip to an amusement park, where they spend two days on rollercoasters and one night in a hotel," Barrett writes.

But whether the babysitter misinterpreted the okay she got to use the parent's credit card may depend on other obvious or even less obvious facts, Barrett said.

"Was the babysitter's trip consistent with the parent's instruction? Maybe in a literal sense, because the instruction was open-ended. But was the trip consistent with a reasonable understanding of the parent's instruction? Highly doubtful."

Barrett wrote that, under one set of contextual facts, the babysitter's move would seem to overreach on an instruction that was meant for a trip to the "local ice cream parlor or movie theater, not on a multiday excursion to an out-of-town amusement park."

But under another set of circumstances -- "maybe the parent left tickets to the amusement park on the counter" or perhaps the parent told there was $2,000 budgeted for weekend entertainment -- the babysitter's move to go the amusement park would seem a "reasonable" understanding on what the parent had approved, Barrett said.

"If a parent were willing to greenlight a trip that big, we would expect much more clarity than a general instruction to 'make sure the kids have fun,'" she wrote.

"In my view, the major questions doctrine grows out of these same commonsense principles of communication," Barrett wrote. "Just as we would expect a parent to give more than a general instruction if she intended to authorize a babysitter led getaway, we also 'expect Congress to speak clearly if it wishes to assign to an agency decisions of vast 'economic and political significance.'"

The Supreme Court's use of the major questions doctrine to throw out Biden's loan forgiveness program struck a nerve that went beyond educational policy -- with the conservative justices and liberal justices sparring over how the controversial legal theory that has been used, with increasing frequency, to strike down unilateral acts of executive branch authority.

Justice Elena Kagan, in a dissent joined by the other liberals, said in a footnote that she could "practically rest" her case for why the loan forgiveness program was lawful on Barrett's "thoughtful" concurrence.

Kagan noted other tests Barrett put forward for interpretation under the doctrine, such as whether the challenged action was in the agency's wheelhouse or whether the delegated power was tucked away in some "ancillary provision. "

"The broadly worded 'waive or modify; delegation IS the HEROES Act, not some tucked away ancillary provision," Kagan wrote, referring to the relevant student loan program statute. "And as JUSTICE BARRETT agrees, 'this is not a case where the agency is operating entirely outside its usual domain.' So I could practically rest my case on JUSTICE BARRETT's reasoning."