The 50x30 mm labels are compatible with the Niimbot B1 label printer. They are specially designed for use with the B1 model as well as other Niimbot label makers such as B21 and B3S and utilise direct thermal printing technology without ink or toner. The labels are usually available in packs of 230, although the 900 labels you mentioned probably comprise several packs. For optimum printing results, the compatibility of the labels with the printer model should be ensured, which is the case with these labels.