Hello,
The on chip reference is 2.5V typ but can vary from 2.3V to 2.7V. The device also has an offset of 7mV typ, 35 mV max and a gain error of 0.1% of span typ, 0.7% of span max. So, there will be some variation from part to part. The on-chip reference has a drift of 50 ppm/degC typ. So, the reference is not highly accurate. An external reference would be lower drift and has better initial accuracy so would give better matching between ICs.
Regards,
Mary.
