Advanced breast cancer is metastatic (Stage IV); meaning it has spread at least to bone, or liver, or brain, or lungs, or a combination. Stage IIIA is still considered early stage; and technically IIIB and IIIC is known as locally advanced.

Ki-67 is not the bigger predicter of breast cancer recurrence, but it is one of many factors. Size of tumor, nodal involvement, and even tumor Grade have much more influence on the risk of recurrence. Stage III are by definition larger tumors; 4-positive nodes is what's contributing to stage III over II (stage II nodal involvement is <4).

If you take a look at figure 1 (Hazard of recurrence among women after primary breast cancer treatment–a 10-year follow-up using data from SEER-Medicare by Cheng published in 2012)

What this is showing you is that Stage III disease (which is larger tumor and more nodal involvement) has a higher risk of recurrence, particularly in the first 2-3 years. The peak is higher comparatively to Stage I and stage II. What your doctor is suggesting is to lower that peak…lower the risk of cancer coming back when that risk is the highest. Technically Verzenio/abemaciclib is not FDA approved for patients with Ki-67 <20%, like you. However the major guidelines for practice (NCCN, ASCO) all endorse adding abemaciclib based on the available data published data shown to reduce recurrence, even in patients with a low ki-67 score but stage III disease like yourself.

Hope this helps.