Hi,
L1C products -> TOA reflectances
L2A products -> BOA reflectances
In both cases, the pixel stores an integer to save space in the disk. For converting these DNs to the reflectances, you have to apply the quantification value. When you open the product in SNAP, this quantification value is applied automatically, but when you open only a jp2 file, you have to apply this quantification value manually.