Hi Antony,
Duration of stay refers to the longest period the visa holder is allowed to stay in China from the date of entry into China each time.
The holder of a visa is eligible to enter China at any time prior to the expiry of its period of validity (allowed to enter China even before the 24:00 hour of the last date of its period of validity) provided that the granted entries on the visa have not been used up. Duration of stay is no longer useful once the period of validity expires.