This is probably a bizarre question, but how does an icon artist go about deciding which icons to make? For example, almost every set seems to include at least

This is probably a bizarre question, but how does an icon artist go about deciding which icons to make? For example, almost every set seems to include at least one airplane, so airplanes seem to be something icon artists expect will be useful... but there are also bathtubs in some sets, which seems odd. Are these collections the results of specific projects, or do they anticipate what designers might need?