It's the filename of the elf file, I think.
For srecords creates with the burner, there is an option -Ns to configure if the full path or just the filename should be written to the s0, or if no s0 should be emitted at all.
However I don't know if this option is already in the V3.0 version you are using. Also srecords can be generated by other tools too (assembler/linker).
Daniel