Unlocked bootloader "can't" be checked I have a Nexus 6p, and I unlocked it's bootloader. There is a message saying that my phone can't be checked for corruption since the bootloader is unlocked. I am a little confused because when I flashed Paranoid Android, the first time it booted into it, the message said that it was corrupt (so far I don't see anything wrong).

How does it know that my phone is corrupt if it can't check for corruption?
It has to do with dm-verity.  Taken from XDA Developers here:


  The problem (it’s a problem if you like root and modifying devices)
  stems from something I pointed out a long time back, when it first hit
  AOSP – the introduction of dm-verity to Android. Verity is a security
  feature, originally found in ChromeOS, designed to provide assured and
  trustworthy computing devices, preventing malicious software from
  modifying a device. Back in Android 4.4, Google announced verity for
  Android, and then all remained quiet. While there has been some
  research into using verity, for the most part, things have been quiet.
  Until now, that is.
  
  With Android 6.0, Google has begun to up their game on device
  security. One of the fundamental requirements for this is to prevent
  the software on a device from being modified without a user’s
  knowledge – while many here at XDA take root for granted, imagine a
  user’s device being rooted without their knowledge or consent, and
  root access being used to steal their data. For this reason, Google
  has started to implement verification of the system partition on some
  devices.


To answer your question of "How does it know..."...any changes to the system partition at boot (or when accessed) could cause one of three warning/error messages.