(CNN) -- Obamacare is again in the legal spotlight before the Supreme Court.
A blockbuster 2012 ruling upheld the constitutionality of the key funding part of the politically charged law aiming to get health coverage to millions of uninsured Americans.
This time, the court is looking at a part of the law that took effect on Wednesday requiring most employers to provide contraception coverage.
A Catholic charity that cares for the low-income elderly, the Little Sisters of the Poor, asked the Supreme Court this week to exempt it from having to comply.
Here are five things to know about the case:
1. The law
The 2010 Affordable Care Act requires non-profit groups and businesses of a certain size to provide health insurance to their employees or pay fines.
One preventive care requirement initially included mandatory contraception -- or birth control -- and other reproductive health coverage with no copay to women.
2. The objection
Some religious groups that include Catholics and evangelicals objected on grounds the requirement violated their religious freedom and moral beliefs.
3. How was it handled?
The Obama administration and religious organizations set out to negotiate how the provision would be carried out. Under rules they came up with last summer, churches and houses of worship would be exempt.
But under the compromise, church-run hospitals, universities, parochial schools and charities must provide no-cost contraception coverage, or have a third-party insurer provide separate benefits without the employer's direct involvement. And most are complying.
Organizations that opt-out of direct coverage must sign a document certifying their religious objections and authorizing payments. The government would then require the outside insurer pick up the costs.
4. What's the core issue now?
A key here is Christian Brothers Services, a Catholic-affiliated non-profit that administers health plans for Little Sisters of the Poor and hundreds of other faith-based groups.
CBS qualifies for an exemption under the Obamacare rules for contraception and so does the Little Sisters of the Poor. All the nuns have to do -- and the Justice Department concurred on Friday -- is fill out the form seeking a pass, and they'll get it.
But they contend in their suit that finding someone else to fulfill the insurance mandate -- even if it is unclear who that might be right now -- is immoral because contraception coverage would be delivered ultimately and they oppose it.
The nuns and the provider on Tuesday asked the Supreme Court to intervene one day before the requirement was due to take effect along with other provisions of Obamacare.
Justice Sonia Sotomayor granted an emergency injunction, pending her review of arguments from all sides. She could extend the injunction temporarily in order to allow for further review, end it without further action, or kick the matter up to the entire court to review.
5. Who would be impacted by a ruling?
Any further court order could ultimately impact scores of religious groups and non-profit businesses, some of which have mounted legal challenges in recent months to the law over contraception.
But it would be only a stop-gap or temporary measure, either blocking or allowing enforcement until the federal courts decide larger legal and constitutional questions.
It's a process that may take many months, perhaps ultimately ending again at the Supreme Court for final review.
Separately, the justices in March will take up a related challenge to the birth control coverage mandate. That appeal concerns whether some for-profit corporations should be exempt, again on religious liberty grounds.
The case, led by owners of the Hobby Lobby retail chain, may have less chance for success than the appeal presented by the nuns.
CNN's Bill Mears contributed to this report.