wolfram mathematica - What is dollar sign $ used for internally? -

what symbol $ used internally?

i not mean compound forms x$388 or $5 etc., $ itself.

i wondering if valid object use in notation, or break if do.

in version 7, symbol system`$

used created in fresh kernel, not used far know. in version 8, symbol $ not pre-created:

in[1]:= context["$"]  during evaluation of in[1]:= context::notfound: symbol $ not found. >>  out[1]= context["$"] 

i agree szabolcs code using $ in system context might break in future versions, other code modifies system symbols.