I am asked for a key / serial number, what should I specify?

The data may be needed by the developer to generate a license. In most cases, this is the key of the current license - to generate a renewal / version change or to provide a discount. This key can be viewed in the program itself or in the registration letter for a previous purchase. You can independently duplicate the letter with the previous license code if it was not saved in the mail.
In some cases, it is required to pre-install the demo version of the program on PC where it will be used in the future. You will be able to see required combination of symbols directly in the installed demo.