EXFO FTB-5230S Optical Spectrum Analyzer for FTB 200 v2 - Manual (Page 143)

Setting Up the Instrument in Drift Mode
OSA/OCA 137
Setting Up Acquisition Parameters
6. Select a location where the drift file should be saved.
7. Select whether you want to keep all of the historical traces in the
subfolder, keep only the significant ones, or keep none. The historical
traces are stored in separate *.osawdm files.
A significant event is when

a value from a given channel has crossed its threshold (going from
pass to fail).

there is no signal power in a given channel.
These historical files are stored in a dedicated folder having the same
name as the associated drift measurement file name.
Note: You can have a maximum of 3 significant traces per channel.
Note: This option is not available in offline mode.