Hi,
the other band (flag) is a band containing information about the validity of the output. It is an integer generated by 5 bits:
A value 0, for example, is obtained when all is right. A value 1 means that one or more inputs are out of range…
From the ATBD (http://step.esa.int/docs/extra/ATBD_S2ToolBox_L2B_V1.1.pdf), these values are:
