Dhivagar,
Please see the SA124 product manual (linked below) in section 2.4 for more information about the IF output. This feature allows the SA124 to be used as a downconverter, and in this case, the 6MHz RBW refers to the bandwidth of the IF output. This should not be confused with the 6MHz sweep RBW available through use of a power meter.
https://relatedcontentdemo.kinsta.cloud/sigdownloads/SA124B/SA124B-User-Manual.pdf
Regards,
Andrew