The grade of service in a TXE2 was dependent on the number of customers in an A-switch group, with access to just 25 A-B trunks. The normal standard on the earlier exchanges was 125 customers per A-switch group. If the A-switch group contained many busy PBX lines, then the number of customers might be reduced to 75. The earlier (Mark I and Mark II – the differences being slight) exchanges could handle up to 2,000 customers. Later on, Mark III TXE2s were able to handle up to 4,000 customers and on these exchanges, where the average calling rate was sufficiently low, up to 250 customers would be in an A-switch group, still with access to just 25 A-B trunks.