Hi Abhishek,
When you are migrating a volume larger than 4TB from an external storage then you need to enable this Data Direct Mapping attribute, below is the reason behind this concept.
Even if the source volume is a Thin Provisioning volume, data in all user areas is copied at the time of data migration. Therefore the pool of the target volume needs to have a free capacity that is larger than the capacity of the target volume. When the source volume over 4 TiB as an external volume. In this case, enable the setting of Data Direct Mapping. When Data Direct Mapping is enabled, LDEV is always created when the volume is mapped.
If you want to know more detail then please refer to the Universal Volume Manager reference guide. Thanks