When you register your details and pay the fee then you receive a private key. This key unlocks the programme from trial version to full version and thus you can use it worry free - because after 21 days it wont stop working and it unlocks certain features that aren't available in the trial.
However there are many websites offering private keys for free, so you dont have to go directly to them (and pay the 20 dollars fee) to unlock the features....
hope that helped !
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