You have to follow both, i.e. stay at most 28 days at some time between 02/08/2015 and 13/09/2015. Therefore, you cannot stay longer than 28 days on this visa in any event and if you have not used your 28 days by the 13th of September, you must leave anyway.
You also have to respect the number of entries, which means that if you enter some time in August and leave a few days later, you cannot use this visa anymore and you lose the benefit of the rest of the maximum period of stay.
09/08/2015 to 05/09/2015 is indeed 28 days and within the period of validity of your visa, so that’s fine.
In principle, the conditions of your visa should be based on the information you submitted with your application and allow you to complete your trip as intended. The validity is typically a little longer to give you some flexibility.
You have to follow both.
You may enter the Schengen area at any time during the 43-day period of its validity, but you may only stay 28 days in total, and you must leave on or before 13/09/2015, even if you have not used all 28 days.
Also, since this is a single entry visa, you cannot leave the Schengen area and return on this visa, even if you have used fewer than 28 days.
However, using the visa properly establishes a good history and makes it more likely that you will receive a double entry or multiple entry visa with longer validity periods in future.
Credit:stackoverflow.com‘