If upgrading from an earlier version of PS then you’ll need a serial number from version 7. You’ll need to contact Adobe for a new format serial number for earlier versions.
Activation REQUIRES and is TIED to a valid serial number. When you install PSCS you insert your serial number and PS then creates machine specific activation code. When you use the Internet based activation routine PS transmits BOTH numbers to Adobe and an automated reply inserts the authorisation code on your machine by return. This only takes a few seconds If using the Phone based system then you’ll need to key in the two numbers before Adobe will give you the authorisation code.
The following might help explain it further
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http://www.computer-darkroom.com/photoshop_cs/activation.htm>