-These three instructions collect the data out of the B<RRD> files,
-optionally altering it (think of a bytes to bits conversion) and
-if desired you can define variables containing useful information
-such as maximum, mininimum etcetera. Two of the instructions use
-a language called B<RPN> which is described in its own manual page.
+These three instructions extract data values out of the B<RRD> files,
+optionally altering them (think, for example, of a bytes to bits
+conversion). If so desired, you can also define variables containing
+useful information such as maximum, minimum etcetera. Two of the
+instructions use a language called B<RPN> which is described in its
+own manual page.
+
+Variable names (I<vname>) must be made up strings of the following characters
+C<A-Z, a-z, 0-9, -,_> and a maximum length of 255 characters.
+
+When picking variable names, make sure you do not choose a name that is
+already taken by an RPN operator. A safe bet it to use lowercase or
+mixedcase names for variables since operators will always be in uppercase.