The If It Fits Your Macros (IIFYM) diet allows for flexible food choices as long as daily macronutrient (protein, carbs, fat) and calorie intake targets are met. It focuses on consuming fewer calories than one's total daily energy expenditure (TDEE) through tracking macronutrient grams using apps and food scales. While the diet is flexible, it recommends basing 80-85% of foods on whole, healthy options and allowing 15-20% for "junk" foods, while also meeting fiber intake guidelines.