Here's how Sloan Kettering and two other sites define Stage 1B. M.D. Anderson uses the same standard. As does my oncologist at Cleveland Clinic. The definition is the same across the industry.

Sloan-Kettering:
Stage I describes a very early stage of invasive cancer. At this point, tumor cells have spread to normal surrounding breast tissue but are still contained in a small area. Stage I is divided into two subcategories:

In stage IA, a tumor measures up to 20 millimeters (about the size of a grape), and there’s no cancer in the lymph nodes.

Stage IB can be described as either:
a small tumor in the breast that is less than 20 millimeters plus small clusters of cancer cells in the lymph nodes; or
no tumor in the breast plus small clusters of cancer cells in the lymph nodes.
https://www.mskcc.org/cancercare/types/breast/diagnosis/stages-breast
Cancer Center:
Stage 1A: The tumor is very small and has not spread to lymph nodes.

Stage 1B: Cancer between 0.2 mm and 2 mm is found in the lymph nodes (the breast tumor can't be detected or is smaller than 20 mm).
http://www.cancercenter.com

BreastCancer.org:
In general, stage IA describes invasive breast cancer in which: the tumor measures up to 2 centimeters (cm) and the cancer has not spread outside the breast; no lymph nodes are involved

In general, stage IB describes invasive breast cancer in which: there is no tumor in the breast; instead, small groups of cancer cells — larger than 0.2 millimeter (mm) but not larger than 2 mm — are found in the lymph nodes

or there is a tumor in the breast that is no larger than 2 cm, and there are small groups of cancer cells — larger than 0.2 mm but not larger than 2 mm — in the lymph nodes
http://www.breastcancer.org