How Does Unix Interpret $?

[orca@orcacomputers public_html]$ adduser Jé$$è+rèè adduser: invalid user name 'Jé28956è+rèè' I am unable to use $ when adding users in CentOS7. Karen goes "WHY THE HELL WOULD YOU DO THAT!?" as she socially crucifies me as I am being my authentic self with is a Left Handed Neanderthal. Curiosity. I am an auditor by trade so I increase my levels of happiness when I curate data. So what is $ telling unix here? What I can is so far is $$=28956 in unix. Is this a random string? Are there any ways to use the $ character to sort data? or any other usecase that would be deemed productive for Computer Science purposes? [orca@orcacomputers public_html]$ adduser Jéssè+rèè adduser: invalid user name 'Jéssè+rèè' What really!? Is this where I have to make my own distribution?