void appendSerialForm(StringBuilder b, boolean includeOrder) {
            if (includeOrder) {
                if (sortOrder == Order.ASCENDING) {
                    b.append('+');
                } else {
                    b.append('-');
                }
            }
            boolean emitMissingFunction = missingValueSettings.hasNondefaultSetting();
            if (emitMissingFunction) {
                missingValueSettings.appendPrefix(b);
            }
            b.append(fieldSorter.toSerialForm());
            if (emitMissingFunction) {
                missingValueSettings.appendSuffix(b);
            }
        }
Returns the sorting order of this attribute
container-search/src/main/java/com/yahoo/search/query/Sorting.java