Reading the codeAT5-500-16 is a AT5 belt with a 500mm length and a 16mm width. Some makers write it as 16 AT5/500 (width, profile, length), which means the same belt.
Counting teethLength divided by 5mm gives the tooth count, so a 500mm AT5 belt has 100 teeth. Check whether you need an endless belt or open length for clamping.
AT5 or T5?Both are 5mm pitch, but AT5 teeth are bigger and need AT pulleys. If the drive has T5 pulleys, stick with a T5 belt.