To customize your Nexus 6P, first you need to unlock its bootloader by executing the command ‘fastboot oem unlock’, but things have now changed with the new Nexus 6P. The old command will not work with the Huawei Nexus 6P and requires the new command ‘fastboot flashing unlock’. At the same time Nexus 5X accepts both the old and new commands.
In addition, there are two different ‘levels’ for unlocking the bootloader on Nexus 6P – basic and critical. For normal customization, such as flashing system images, custom recovery, custom ROM, boot images, etc. ‘basic’ level (
fastboot flashing unlock) is sufficient.
However, for crucial operations like flashing custom bootloaders needs the “critical” level (
fastboot flashing unlock_critical). Critical level will remove any security restriction.
The new unlocking/relocking commands includes:
fastboot flashing unlock
fastboot flashing lock
fastboot flashing unlock_critical
fastboot flashing lock_critical
fastboot flashing get_unlock_ability
Before unlocking the bootloader, keep in mind that the process will void your Nexus 6P manufacturer warranty. Also, you must be careful to back up all of your apps and files, because unlocking the bootloader will wipe the lot from your device.
Note that the bootloader is an important part of your Android OS. It is a security feature that tells your phone what it should load. That is, it allows running programs that have been verified by the bootloader. So, unless you have a specific reason, always relock the unlocked bootloader. (Source – AP)