During development, the KDE community sets goals in features and dates for upcoming releases of various projects. This applies for the bigger projects, like Frameworks, Plasma or KDE Applications. This way, the teams 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
release-team mailing list.
See also the (even if outdated) Release Schedule Guide.
See also the release calendar maintained in Phabricator
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.