Initialize:

This option is available when creating an iSCSI (I) or Fibre Channel (FC) volume. It is here for security reasons. The volume will be initialized after the creation process. There can be only one volume being initialized at any one time. If there are more volumes to be initialized, a Waiting indicator will appear near the one(s) enqueued. Using the button next to the indicator you can send the initializing volume to the back of the initialization queue. Every volume that is waiting for initialization in queue can also be sent to the back of queue.

Initialization is made for data protection. Basically when you initialize your volume you fill it with zeros, then when you write over the volume the space that has no data will be protected.

Its a low level format, and many big companies are using this, EMC for instance force you to do it. It’s a good feature. It may take time for big volumes, but it’s so important..