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)
