This tutorial deals only with how to add an extra encrypted physical volume to a volume group pool containing other encrypted physical volumes. This is typical scenario if, at first, you have set up your encryption at a physical partition level (/dev/sdaX where X is the a number of your partition), then you setup your LVM on top of the encrypted partition. If at some later time you want to add another partition in your volume group, you will also want to have it encrypted in order to maintain the same level of security.