what is the -s option used for?

[SusanaFreitas]

Sorry if this was explained somewhere else, but I couldnt find the information anywhere.
I understand the usage of the readgroup, but I don't understand what is the -s for. When adding a new sample name and reagroup (with options -s and -r, respectively), the bam file will only keep the readgroup I added, and no sample name.
So, what does it do?

Thanks!