Hi Mattia,
the .exposure and .meas tables share a column 'exposure', which is identical. It encodes several things (please see
http://adsabs.harvard.edu/abs/2018AJ....156..131N and
https://datalab.noao.edu/nscdr1/index.php ), and also (as the very first part of the string) the instrument that produced the measurement. In addiiton, .exposure also has a column "instrument".
Hope this helps, ciao!
Robert