An interesting case on grocery items is that solid things that you pour (breakfast cereal, flour, sugar, salt, for example) the nutrition label gives the servin

An interesting case on grocery items is that solid things that you pour (breakfast cereal, flour, sugar, salt, for example) the nutrition label gives the serving size in a US customary volume measure and in a metric mass measure. For example my breakfast cereal label says a serving is 1 cup or 45g. Liquids seem to give a US customary volume and a metric volume. For example the milk I put on that cereal says a serving is 1 cup or 240mL. Checking some sauces I have on hand thick liquids (ketchup and sriracha) are US volume and metric mass like pourable solids and thin liquids (like soy sauce) are both US and metric volume like milk. Butter is also US volume and metric mass. A little bit of research says that this is actually regulatory. For things where how much you can fit in a given volume varies quite a bit depending on how you pack it the FDA requires that the metric units on the label be mass units, and that nutritional information is for that amount of mass of the thing. The actual amount of flour in a cup of flour for example can vary quite a bit depending on how much air got in when you poured it. If the label says it is 110 calories for 1/4 cup (30g) and you want to actually use 440 calories of flour in something you should measure out 120g rather than 1 cup. For things like milk 1 cup is going to have the same amount of milk no matter how you pour it, so they use volume measurement for both customary and metric.