The spdb file is formed by processing the txt files located in the speedcam folder.
The software does not "read" the txt files, but the spdb file. The spdb file is updated every time the navigator is powered up with the files it finds.
If you delete some txt files but do not delete the spdb file, you will see that the spdb file has changed its "weight".
If you delete all the txt files and leave the spdb file, you'll see that the spdb file will also disappear.
If there is a spc file in the speedcam folder but no txt file, the spdb file is not formed.

This happens with Israel Nextgen, perhaps Primo and the old versions work differently...... but I look ahead......