During development, the KDE project sets goals in features and dates for upcoming releases. This way, the team knows when it would be a good time to add a new feature or when it's time to focus on cleaning up any bugs in preparation for a release. Any plans are tentative schedules and the final dates are generally decided on the kde-core-devel mailing list.
See also the Release Schedule Guide.
KDE3 and prior use a different naming scheme (not "SC") and have have all components released together. Support for KDE 3 is discontinued as of 2010.