"Unallocated space" refers to a region on a computer's hard drive that has not been assigned or used by any specific file or program. It is essentially empty space waiting to be allocated or assigned for storage or data usage. Full definition
As such, the evidence is more likely to consist of bits of data in the cloud or unallocated space on your client's hard drive than in a central file room. (lawyerist.com)
After the process completes you'll see the new Unallocated space. (howtogeek.com)
By default, it will take up all the available unallocated space, so just click Apply to create a new partition using all that space. (howtogeek.com)