+
+A source may include a mix of numeric and string data values. When a
+source includes any string data, the data values that are strings are
+set to SYSMIS in the @code{numeric-series}, and @code{string-data}
+follows the @code{numeric-data}. To reliably determine whether a
+source includes @code{string-data}, the reader should check whether
+the offset following the @code{numeric-data} is the offset of the next
+series, as indicated by its @code{metadata} (or end of file, in the
+case of the last source in a file).
+
+@code{string-data} repeats the name of the source.
+
+The string data overlays the numeric data. @code{n-string-series} is
+the number of series within the source that include string data. More
+precisely, it is the 1-based index of the last series in the source
+that includes any string data; thus, it would be 4 if there are 5
+series and only the fourth one includes string data.
+
+Each @code{pair-series} consists a sequence of 0 or more pairs, each
+of which maps from a 0-based index within the series @code{i} to a
+0-based label index @code{j}. The pair @code{i} = 2, @code{j} = 3,
+for example, would mean that the third data value (with value SYSMIS)
+is to be replaced by the string of the fourth label.
+
+The labels themselves follow the pairs. The valuable part of each
+label is the string @code{s}. Each label also includes a
+@code{frequency} that reports the number of pairs that reference it
+(although this is not useful).