PayPal - User Agreement - Seller Protection Policy
"When a seller receives a payment through PayPal, if the buyer files an unauthorized transaction claim or a claim under PayPal’s consumer protection programs (a “Buyer Claim”), or if the buyer files a credit card chargeback, a Temporary Hold will be placed on the amount of the payment. If the transaction qualifies for the Seller Protection Policy, the temporary hold will be lifted from those funds.
If the transaction does not qualify for the Seller Protection Policy, PayPal will review the Buyer Claim, including all information provided by the seller, and make a determination. In the case of a chargeback, PayPal will determine whether the chargeback can be disputed with the card-issuing bank. If PayPal disputes the chargeback, the Temporary Hold will continue until the dispute is decided by the applicable card company (Visa, MasterCard, American Express or Discover)."