A “raw” scan is the data just as it came from the scanner. It is important to archive the raw data so that it can be reprocessed later by more advanced software or a more experienced curator, without any earlier “fixes” getting in the way.
Raw scans should be saved in .TIFF or .DNG format.
TIFF and DNG are both open standards and so both should be future-proof.
The TIFF format is older and so TIFF files can be read by more software, but you need a good photo editor such as Photoshop to read “raw” TIFF files properly including calibration data.
DNG is a newer, more semantic format, has better compression, and is native to Photoshop.
If in doubt, save to both formats!