Thin Provisioning provides a volume size as specified but uses physical storage only when needed. While this sounds like a great way to efficiently use the available storage space, in real life situations this may not always be true. When files are deleted or moved the now unused data is usually still residing on the physical storage. The host system may indicate that the used capacity is now smaller but the physical storage is still in use and remains allocated to that host system.
Space reclamation deals with this situation by providing a way for the host system to indicate that the physical storage assigned to it is no longer in use. space reclamation in operating systems are typically achieved by invoking the Small Computer System Interface (SCSI) WRITE SAME or the SCSI UNMAP command. Which commands are used depends on the operating system involved.
Considerations for using Thin Provisioning with Microsoft Windows 2012 are summarized in the Plan and Deploy Thin Provisioning article published by Microsoft