leagues and the sixth-highest of any professional association football league in the world. CSL has two transfer windows—the primary pre-season transfer window lasts two months from January to February, and the secondary mid season transfer window runs one month from mid June to mid July. As of the 2018 season, the CSL introduced new rules mandating that each club must register a maximum 31-man squad, with 27 Chinese Players, including a player from Hong Kong, Macau and Chinese Taipei, and 4 foreign players. In the transfer window clubs could sign 5 Chinese players at any age, plus 3 under 21 Chinese players; clubs could register 4 foreign players in the winter transfer, and replace two of them in the summer transfer. The record transfer fee for a CSL player has risen rapidly since the investment boost started in 2015. The six most expensive transfers with players coming to CSL have exceeded €30 million, with Chelsea selling Oscar to Shanghai SIPG in December 2016 for a fee of €60 million, Zenit Saint Petersburg selling Hulk to Shanghai SIPG for €55.8 million in July 2016, Shakhtar Donetsk selling Alex Teixeira to Jiangsu Suning for €50 million in February 2016, Atlético Madrid selling Jackson Martínez to Guangzhou Evergrande for €42 million in February 2016, Villarreal selling Cédric Bakambu to Beijing Guoan for €40 million in February 2018, Atlético Madrid selling Yannick Carrasco to Dalian Yifang for €30 million in February 2018. Guangzhou Evergrande's sale of Paulinho to Barcelona for €40 million in 2017 broke the record for a CSL player transfer to other leagues. Transfer fees for domestic players also increased dramatically. Beijing Guoan sold Chinese International Zhang Chengdong to Hebei China Fortune for ¥150 million in January 2017, breaking the domestic transfer record for Chinese players. The Chinese Football Association introduced a new transfer tax to restrict transfer spending. On June 20, 2017, CFA announced that any club that pays more than ¥45 million for a foreign player transfer or ¥20 million for a Chinese player transfer must pay the same amount to a CFA youth development fund. In December 2020, the CFA imposed a salary cap on the Super League. Starting with the 2021 season, total player wages are capped at ¥600 million, with a separate limit of €10 million for foreign players. Individual player salaries are also capped, at ¥5 million before tax for Chinese players and €3 million for