It is not possible to specify an unnamed variable - i.e. a variable that was not
previously specified using Insert|Fields other|Variables|Set Variable - in a
document in the "show variable" dialog. Although this may seem logical, when
working with Master documents (which hold the variables) it creates some
maintenance issues, since it's not very practical to have to set the variable in
each subdocument. Conversely, it IS possible to specify unnamed variables in
conditional text, hidden paragraphs and the like. 

Example : I have a master document with variable "A", which equals 1. In a
subdocument, which does not have variable "A", I place conditional text. The
condition is : 

if (A==1) text="this"; else text = "that"; 

This works perfectly when the conditional text is evaluated from within the
master document. But, using a similar approach, I cannot manually specify "A" in
the "show variable" dialog from within the subdocument.