What is the meaning of $ in a IFC definition?

This is a snippet of IFC definition:

#20=IFCORGANIZATION('','Anon Consulting','',$,$);

What are the meaning of the $ characters? For now, my educated guess is that they represent `null. Where can I verify this?

Solution

BuildingSmart (owner of the IFC standard) says

The $ character encodes the indefinite value, i.e. it appears if an optional attribute has not been filled in.

So, yes, it is essentially a 'null' value.