Understanding how the space reservation setting (combined with the volume guarantee) affects how space is set aside for LUNs helps you to understand the ramifications of disabling space reservations, and why certain combinations of LUN and volume settings do not make sense to employ.
When a LUN has space reservations enabled (a space-reserved LUN, the default setting), and its containing volume has a volume guarantee, free space from the volume is set aside for the LUN at creation time; the size of this reserved space is governed by the size of the LUN. Other storage objects in the volume (other LUNs, files, Snapshot copies, etc.) are prevented from using this space.
When a LUN has space reservations disabled (a non-space-reserved LUN), no space is set aside for that LUN at creation time. The storage required by any write operation to the LUN is allocated from the volume when it is needed, provided sufficient free space is available.
If a space-reserved LUN is created in a none-guaranteed volume, the LUN behaves the same as a non-space-reserved LUN. This is because a none-guaranteed volume has no space it can allocate to the LUN; the volume itself can only allocate space as it is written to, due to its none guarantee. For this reason, creating a space-reserved LUN in a none-guaranteed volume is not recommended; employing this configuration combination could seem to provide write guarantees that are in fact impossible.