Glossary
Unallocated space
Clusters or blocks a filesystem is not currently using. Often still holds the contents of deleted files, which makes it the primary surface for carving.
Unallocated space is every cluster the filesystem's allocation structure marks
as free: the NTFS $Bitmap, the FAT table, the exFAT allocation bitmap, the ext
block-group bitmaps, the HFS+ Allocation File. Free does not mean empty. When a
file is deleted, most filesystems only flip its allocation bits and unlink the
directory entry. The data sits there untouched until something reuses the
clusters.
That is why unallocated space is where deleted files are recovered from. There is no metadata pointing at the content anymore, so you fall back to carving by signature: scan the bytes for known headers and pull files out by hand.
The parser concatenates a volume's free clusters into one extractable
[unallocated space] object, the same surface FTK Imager exposes.