https://www.reddit.com/r/qnap/comments/3prskw/understanding_qnap_storage_manager/

A storage pool is a group of disks, usually best to use identical disks otherwise you risk losing space on the larger drives. Your apps and users never have access to the storage pool, it is internal only on the NAS.

A volume must be contained within a storage pool, this is what your users and apps will be accessing. A storage pool can contain one to many volumes. A volume can only have one storage pool.

QNAP at some point (I think 4.2 but I don't mess with volumes very much so I'm not sure) introduced three types of volumes:

Static Single Volume - this is fixed size and all the space is assigned on the volume at once, this is the classic mode, so if you assign 1000Gb volume it will use 1000Gb straight away regardless of how much space is used Thick Multiple Volume - so this is where it gets a little complicated, say you have 1000Gb storage pool, say you want to create a volume but not use all that 1000Gb space today but you might want it later, so you create a thick volume pool. You set the max it can grow to, anything up to 1000Gb and a starting size, say 100Gb. That means the volume will straight away consume all that 100Gb space but NOT the 1000Gb. As you add data over the 100Gb it will get larger and larger. Thin Multiple Volume - same as previous but there is no minimum size.

The benefits of the second two options above is that you can over commit the storage pool. So I could setup three volumes all 500Gb in my 1000Gb storage pool, this is fine because you would then add more disks once you started getting close to the max capacity of the storage pool. However this is usually pointless on a 4 bay NAS as all 4 bays are normally in use, but regardless it is a useful feature for NAS with a lot of bays, I frequently use it at work with big SANs. It is also why the max volume is ~64Tb, as that is the virtual limit for the storage pool on your hardware and nothing to do with the physical limit imposed by your disks.

The second two options also allow for snapshot backups, which may or may not be useful to you, but these also consume disk space from the storage pool independently from the volume.

You have chosen thin, so the volume will only be sized to what is actually in use, free space will be set to whatever you set the max for that volume. This is solely down to the way you set up the volume and not a bug or a problem.

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