A gene from a soil bacterium, for instance, makes soybeans immune to a widely used weed killer. A type of papaya has been modified to be resistant to a crippling virus, and corn has been altered to control insects that attack it.

Only a handful of such crops are grown around the world. But some of them, like corn, soybeans and sugar beets, are used in the majority of processed foods that line the aisles of grocery stores, including chips, soda, salad dressing, soups, some breakfast cereals and baked goods. So a lot of foods items stand to bear these labels.

Here, by the way, is a list of common misconceptions about what constitutes a G.M.O.

Why are these guidelines being proposed now?

Major food manufacturers have fought long and expensive battles against G.M.O. labels, worried that they would deter customers — and give an advantage to organic food makers, who lobbied to advance labeling legislation and ballot measures in several states. (Under U.S.D.A. guidelines, organic foods may not contain genetically engineered ingredients.)

In recent years, legislators and consumer groups have stepped up efforts to pass labeling laws, with bills or ballot initiatives appearing in California, Connecticut, Maine, Oregon and Washington. After Vermont became the first state to pass a labeling law, in 2014, food makers faced the expense and logistical hurdles of retooling their packaging for one market — with others potentially on the horizon.

Some manufacturers announced that they would voluntarily place labels on their genetically engineered foods nationwide. Others agreed to push for legislation that would impose a nationwide standard — with provisions like a QR or bar code that consumers could scan with mobile phones, and a term, “bioengineered,” that they preferred. Those efforts turned into a federal labeling bill signed into law by President Barack Obama in 2016. The U.S.D.A. then had to determine just what those labels would look like.