FSET and FRSET both are the concepts related to the BMS.
FSET is Fieldset is used in the ATTRB parameters of the field definition of the BMS maps(DFHMDF)
FRSET is Field Reset is used in the CNTL parameters of the map definition (DFHMDI) or mapset definition(DFHMSD)
FSET sets to the related flag bit (7th bit of Attribute character) to 1.
if you dont give this in the attrb of the dfhmdf the seventh bit( or last bit) of the attribute character is set to zero this means that the particular field is not modified by the end user, and if the enduser is entered some data in that particular field this bit will be set to 1, with out giving the FSET in its attrb parameters.
this option is very useful in setting the MDT to set on so that while receiving the map all the fields which have been given FSET are recived irrespective of their modification at the terminal by the end user so that the mapfail condition could not occur.
FRSET does nothing but resets all the fields attribute characters of the maps to 0.