Just did a search in the whole GeoTools, it is used only in its test, and mentioned (without links) in a couple of javadocs:

````
./modules/library/main/src/main/java/org/geotools/filter/visitor/PostPreProcessFilterSplittingVisitor.java: * the old SQLUnpacker class, which did the same thing using recursion in a more straightforward
./modules/library/main/src/main/java/org/geotools/filter/visitor/CapabilitiesFilterSplitter.java: * works, compare it with the old SQLUnpacker class, which did the same thing using recursion in a
./modules/library/jdbc/src/main/java/org/geotools/filter/SQLUnpacker.java: * the clients responsibility to deal with that.  The SQLUnpacker should be
./modules/library/jdbc/src/main/java/org/geotools/filter/SQLUnpacker.java: *       static  methods SQLUnpacker.getSupported(Filter, splitType,
./modules/library/jdbc/src/main/java/org/geotools/filter/SQLUnpacker.java:public class SQLUnpacker {
./modules/library/jdbc/src/main/java/org/geotools/filter/SQLUnpacker.java:    public SQLUnpacker(FilterCapabilities capabilities) {
./modules/library/jdbc/src/test/java/org/geotools/filter/SQLFilterSuite.java:        suite.addTestSuite(SQLUnpackerTest.class);
./modules/library/jdbc/src/test/java/org/geotools/filter/SQLUnpackerTest.java: * Unit test for SQLUnpacker.
./modules/library/jdbc/src/test/java/org/geotools/filter/SQLUnpackerTest.java:public class SQLUnpackerTest extends SQLFilterTestSupport {
./modules/library/jdbc/src/test/java/org/geotools/filter/SQLUnpackerTest.java:    private SQLUnpacker unpacker;
./modules/library/jdbc/src/test/java/org/geotools/filter/SQLUnpackerTest.java:    public SQLUnpackerTest(String testName) {
./modules/library/jdbc/src/test/java/org/geotools/filter/SQLUnpackerTest.java:        LOGGER.finer("running SQLUnpackerTests");
./modules/library/jdbc/src/test/java/org/geotools/filter/SQLUnpackerTest.java:        TestSuite suite = new TestSuite(SQLUnpackerTest.class);
./modules/library/jdbc/src/test/java/org/geotools/filter/SQLUnpackerTest.java:        //Set capabilities for the SQLUnpacker
./modules/library/jdbc/src/test/java/org/geotools/filter/SQLUnpackerTest.java:        unpacker = new SQLUnpacker(capabilities);
````

Searched in the GeoServer code base, no reference at all. Maybe uDig?