PCT is a parameter in DMARC that allows domain owners to specify the percentage of messages that fail DMARC that should be subjected to the policy. For example, a PCT value of 50 means that half of the email messages that fail DMARC should be treated according to the DMARC policy (such as being rejected or quarantined), while the other half should be allowed to be delivered as usual.

While the PCT parameter is part of the DMARC standard, it is up to the receiving email server to decide whether or not to honor it. Some receiving email servers may choose to honor the PCT value, while others may not. In practice, the PCT parameter is often not used or is set to a low value because it can be difficult to accurately determine the percentage of email that should be subjected to the DMARC policy, and because some email servers may not honor it.

It's worth noting that even if the PCT parameter is not honored by a receiving email server, the other components of the DMARC policy (such as the "p" parameter and the "rua" and "ruf" tags) can still be effective in protecting your domain from email spoofing and phishing attacks.

It's important to note that the PCT component is not a guarantee that a certain percentage of messages will be blocked, but rather a suggestion to receiving servers on how to handle messages that fail DMARC checks. Some receiving servers may not be able to comply with the sender's suggested PCT due to technical limitations, so it's important to carefully consider the potential impact of setting a PCT value in your DMARC policy.