Last week, just before they adjourned for summer recess, Congress passed a bill that will establish national standards for labeling food containing ingredients which are genetically engineered (GE) also known as  genetically modified organisms (or GMOs). President Obama is expected to sign the bill into law in the coming weeks.

Does this mean U.S. shoppers will soon see labels disclosing GMO ingredients on all food products? Not exactly.

The bill directs the U.S. Department of Agriculture (USDA) to begin the process of deciding what exactly food manufacturers will be required to label. It will be up to USDA to define which ingredients count as “genetically modified ingredients” for the purposes of the law. The agency is supposed to complete this process within two years. And, as Consumers Union senior scientist Michael Hansen told Civil Eats, exactly what will be labeled “depends on how the definition gets interpreted.”

Here’s what you need to know.

What will be labeled?

The short answer is: It depends on who you ask. But as USDA’s General Counsel explained in a letter to Senator Debbie Stabenow (D- Michigan), one of the bill’s lead sponsors, the bill gives USDA “the authority” to include in its definition of GE food, all commercially grown GMO corn, soy, canola, and sugar from sugar beets. That could also mean that any food made with the oils and sugars—including high-fructose corn syrup—made from GMO corn, could be included in labeling requirements.

Hansen, however, thinks the language in the bill “clearly exempts highly processed foods” that typically contain these oils and sweeteners. But Environmental Working Group (EWG)’s senior vice president for government affairs Scott Faber disagreed. “The clear intent of the law is to require labeling of all food derived from GE ingredients,” he said.

Like Consumer Union’s Hansen, Friends of the Earth and the JustLabelIt campaign are concerned about how the bill describes bio-engineered food. They point to language describing it as food that contains genetic materials created through “in vitro recombinant…DNA techniques.”

This, argues Friends of the Earth, would exclude many GMOs. Among the group’s concerns about only labeling food to “contain” GE material, is how the labeling rules USDA will develop will treat food that includes GMO ingredients, but in which it’s not possible to detect the modified DNA. This has yet to be determined.

For example, says Center for Science in the Public Interest (CSPI) biotechnology project director Greg Jaffe, some “highly refined products” like soda “don’t have any of the genetic engineered DNA” in the finished product. USDA could also decide to exempt products that contain only a small fraction of GMO ingredients.

Another concern is that the labeling rules could exclude food like genetically engineered salmon, which is modified using genes found in nature rather than those entirely engineered in a lab. Another concern is that the bill would exclude certain engineering techniques. But as USDA wrote to Senator Stabenow, the agency believes the bill gives it the authority to include foods that includes “novel gene editing techniques such as CRISPR” in labeling requirements. (CRISPR is a genome editing tool that enables geneticists and medical researchers to edit parts of a genome by cutting out, replacing, or adding parts to the DNA sequence.)

Some advocates are also concerned by the bill’s language specifying that to be considered a GMO food, the modification would have to be one that could not be “otherwise obtained through conventional breeding or found in nature.” Friends of the Earth’s food and technology policy campaigner Dana Perls says this could leave out Roundup Ready corn and soy and some widely grown Bt crops. However, the USDA has said the bill does give the agency the authority to include these crops.

“One very positive part of this legislation is that it covers thousands of products that were exempted from the Vermont law (and would be exempt from other state laws),” notes CSPI’s Jaffe. Unlike Vermont’s GMO labeling bill, the newly-passed bill could include products that contain meat, whether or not the meat itself is genetically engineered.

How will the foods be labeled?

The bill will allow producers to choose what type of label they use. It can be a simple text label, like those that already appears on food packages, or a QR code that directs consumers to the company’s website when scanned with a smartphone.

The fact that the bill does not require what JustLabelIt calls “a simple, at-a-glance GMO disclosure on the package,” has prompted an outcry from Democratic Senators, such as Vermont’s Bernie Sanders and congressional Representatives, including Maine’s Chellie Pingree. And on July 14, Reverend Jesse Jackson wrote a letter to President Obama criticizing this part of the GMO bill as discriminating against the 100 million Americans—most of whom are low income and people of color—who can’t afford smartphones.

The option to use a QR code also raises questions about how fresh and unprocessed genetically engineered food—such as salmon or non-browning apples—will be labeled. This hasn’t yet been specified, but CSPI’s Jaffe pointed out that “the bill does allow for the use of a symbol, which could be put on the produce sticker.”

As for salmon, that too has yet to be determined—and it’s possible that the information will have to be included on a shelf tag.

How will the bill impact other state-level GMO labeling laws?

As soon as this labeling bill is signed into law, it will preempt all state-level labeling laws. That includes Vermont’s, which went into effect on July 1. It will therefore also preempt bills passed in Connecticut and Maine, which required contiguous states to pass similar bills. Technically it won’t affect Washington’s and Alaska’s laws, which require certain farmed fish to be labeled in order to set it apart from wild fish. But Alaska’s Senator Lisa Murkowski has said the existing GMO bill won’t do enough to distinguish GE salmon from wild salmon.

Yet one thing the bill will not do is interfere with any voluntary labels, including those added to products recently by manufacturers such as Campbell’s, ConAgra, General Mills, and Kellogg’s.

The bottom line

Until the USDA completes its process of deciding which foods deserve labels, not much will change for consumers, who will continue to rely on the organic label, manufacturers’ voluntary labels, and retailers’ programs—such as Whole Foods’ decision to avoid selling GE salmon and to provide GMO ingredient information on all products—to find out what’s in the products they buy.

Meanwhile, advocates like EWG, will continue to push to for readable, on-package labels that work for all consumers, whether or not they have smartphones.