BCPM stands for Biology, Chemistry, Physics, and Math. AMCAS recalculates it independently from every course you've ever taken in those four categories, including any repeats — and unlike many undergraduate grade-forgiveness policies, both the original grade and the retake grade count toward this number.
Admissions committees look at sGPA separately from cumulative GPA because it more directly predicts how you'll handle the preclinical curriculum's pace and content. A strong sGPA can meaningfully offset a weaker cumulative GPA, and the reverse is also true, so it's worth knowing your own BCPM number specifically rather than only tracking your overall GPA.