RSA3000-BW1 is an optional RBW (resolution bandwidth) upgrade from 1 Hz to 10 MHz for Rigol RSA3000 series spectrum analyzers.
When you buy the RSA3000-BW1 upgrade, you get a code that unlocks the specified resolution bandwidth range.
Rigol RSA3015N Spectrum Analyzer, 100 kHz - 1.5 GHz
Rigol RSA3030N Spectrum Analyzer, 9 kHz - 3.0 GHz
Rigol RSA3045N Spectrum Analyzer, 9 kHz - 4.5 GHz