The South Korean football league system contains two professional leagues, a semi-professional league and amateur leagues for Korean football clubs.
The highest level of football in South Korea is the K League Classic which was founded in 1983. K League Challenge was founded in 2013 and is currently a second division. Below the level of the professional leagues is the semi-professional National League, which serves as the third division. The amateur K3 League, which is being considered the fourth division, was added to the setup in 2007.
National League and K3 League are called the third division and the fourth division for convenience, but they are disrelated and individual leagues.
There was no avenue for progression between any of the leagues until 2012, when the K League Challenge was founded.
South Korean football league system Wikipedia
Korea Semi-Professional Football League was a semi-professional football league between corporate teams in South Korea from 1964 to 2002. In 1983, with the establishment of K-League, Korea Semi-Professional Football League became the second tier and remained until 2002 before Korea National League, at that time called K2 League, was officially founded in 2003. K League Challenge was created in 2013. Fourth tier K3 League was founded in 2007, and renamed to K3 League Advanced. Fifth tier K3 League Basic was founded in 2017.
The K League was created in 1983 and remained the only officially organized league competition in the country until the creation of the National League in 2003. The K3 League was created in 2007, and divided into two leagues starting from 2017.* As of 2017 season
Below this league structure there are many amateur, university and youth level competitions at varying levels. From 2008, the U-League, a league competition for university football clubs, will begin though it is likely to remain completely independent from the league structure above.
All K League and National League sides qualify for the Korean FA Cup tournament. The top four sides from the Challengers League gained qualification to the next season's FA Cup tournament. Additional qualifiers come from amateur and university level.
The League Cup competition is open to K League teams only, whilst sides from the National League can compete in the National Championship. From 2011, Challengers Cup competition is open to Challengers League teams.
At present, four South Korean sides qualify automatically for the AFC Champions League. Three top teams from K League Classic automatically gains entry to the AFC Champions League. The Korean FA Cup winners also qualify for the AFC Champions League only. However, if the champions of Korean FA Cup are not members of K League, the fourth place of K League Classic the entry.