A DITA2Go macro variable name looks like a DITA2Go macro name, except that a macro variable name starts with two dollar signs instead of one: $$varname. The rest of the name must consist only of letters and digits. Do not include punctuation or spaces in a macro variable name.
Reserved naming for predefined macro variables
Some macro variable names are predefined by DITA2Go, and cannot be used for other purposes; see §37.3.4 Using predefined macro variables. The name of a predefined DITA2Go macro variable starts with two dollar signs followed by an underscore: $$_varname. Avoid giving a name that starts with an underscore to any of your own macro variables; the DITA2Go definition takes precedence. The “$$” says “this is a DITA2Go macro variable”; the “_” says “the name is reserved, not one of yours”.
A DITA2Go macro variable is not the same as a DITA2Go variable, though they share the same naming convention. A DITA2Go macro variable overrides a DITA2Go variable of the same name. See §12.1 Understanding how DITA2Go user variables work.