maybe it is not communicated well in the official documentation, but please don’t blame others who are trying to help because you did not find it. It is mentioned in many documents and it is called the quantification value. It is basically a scaling constant which allows to store the products as integers instead of float to reduce the file size required to store it on a hard drive.
For Sentinel-2 it is 10000.
official source in the Sentinel-2 handbook
source: https://sentinels.copernicus.eu/documents/247904/685211/Sentinel-2-Product-Specifications-Document
practical explanation in this paper
source: https://www.preprints.org/manuscript/201610.0078/v1/download
also applied/documented in secondary platforms, e.g. the Google Earth Engine
source: Sentinel-2 MSI: MultiSpectral Instrument, Level-1C [deprecated] | Earth Engine Data Catalog | Google for Developers
accessing it in SNAP
You find it in the metadata of each product
and it can also be retrieved for each band in the information tool:
