Complex sentence: The MNC can be either two digits long (normal e.g. in Europe) or three digits long (normal in North America), the remaining digits, up to the maximum length are the mobile subscriber identification number (MSIN) within the network's customer base.
Split into simpler sentences: The MNC can be either two digits long (European standard) or three digits long (North America standard). The remaining digits up to the maximum length are the mobile subscriber identification number (MSIN) within the network's customer base.