Visa validity defines the dates/ amount of time that the visa is valid for. In cases where you need to specify the dates and the visa is issued with the trip dates stated on it, that would be the validity of the visa. In these cases the validity of the visa will be the same a duration of stay (the amount of time you are allowed to stay in the country for). So, if the visa is valid from the 1st to the 30th of October, that will be both validity of the visa and duration of your stay.
In other cases, when the visa is valid from the date of issue like China (valid for 3 months from the issue date), 3 months will be the visa validity. You may then be allowed to stay in the country for 30 days only, which will define the duration of stay. So even if the visa is valid for 3 months, you can only stay in China for 30 days.
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