This (numeric) field is used to determine a symbol's asset class.
Asset classes, and example RICs (Reuters Instrument Codes):
Note: a chain is a RIC containing a list of RICs.
FID (Field IDentifier) 259 provides information about an instrument's asset type. It was previously listed in 3000Xtra in C:\Program Files\Reuters\Common\RealTime\appendix_a.
In Eikon it is listed in the real time fields in PL_appendix_a in C:\Program Files\Thomson Reuters\TRD 6\Config, In Eikon 4, C:\Program Files (x86)\Thomson Reuters\Eikon\X\Config OR C:\Program Files (x86)\Thomson Reuters\Eikon\Z\Bin\Apps\TR.OFFICE.CORE\0.0.0.0\Config