How does NMEA collecting work?

I have a RUTX12 in my RV with a GPS antenna on the roof. I have activated NMEA forwarding to a local device, it is working fine.
I have enabled NMEA collecting in Services → GPS → NMEA → NMEA Collecting and choose /mnt/gps as file path.
The file is indeed created but after a few days of travel, the file is still empty. Is there something else to do to activate this feature or are there things that could prevent it to work ?

Nevermind. After opening the NMEA sentences tab and activating sentence names in the self explanatory “Collecting enabled” column, it started collecting.