Phosphorus is watched for kidney disease (CKD) plans. A particular concern is added phosphates — the phosphate additives in many processed foods, which are absorbed much more readily than the phosphorus naturally in food and can raise phosphate levels.
So VitalPlate flags phosphorus two ways: as a nutrient over your per-serving limit, and as an ingredient when it spots added phosphates on the label (see Why a food gets flagged). CKD plans also track a daily phosphorus ceiling (around 900 mg is commonly used).