The account key is a key that is generated for your account. This key is protected by your password and used to encrypt files that are decryptable on any of your device(s) or your web browser.
This is the most secure encryption key level as it is only stored securely on your individual devices. This allows for you to have files to be encrypted for individual devices. Even if is your password is known by a hacker, the files cannot be decrypted without the device in their possession. Unfortunately, this means that your files are not recoverable if the device is lost or destroyed. Please also see Can I Lose my Files?.
Secure File Deletion is used for when a sensitive file needs to be completely wiped off a HDD or SDD. When a user deletes a file off a computer in the standard manner, it leaves residual information on the hard drive, which can be used to reconstruct the original file. The Secure File Deletion algorithm that EZ-N-Secure uses is known as DoD-7, which writes random data to the location of the file, overwriting the residual information. This makes recovering this file extremely difficult as most of the residual information will be destroyed.