Every foreign visitor wishing to enter the Republic of Korea (South Korea) must have a Korean visa, or the visitor's country of residence must have a visa-free agreement with the Korean government. You can find more details through the Korean government website.
An official invitation letter will be sent to a registrant upon request. If you need an invitation letter for participating in ACM CCS 2021, please download and fill out the Invitation Letter Application form , and them send email at email@example.com, the local secretariat. An official invitation letter will be sent only when the one who requested have registered for the conference and completed the payment. Note that this invitation implies no obligationfor any visa.
Nationals of those countries or regions with which Korea has signed a visa waiver agreement can enter without a visa, on the condition that they do not engage in remunerative activities during their stay. (Up to 90 days unless stated otherwise)
The 107 countries listed on the website linked here are under a Visa Waiver Agreement with Korea. Depending on nationality and passport type, citizens from these countries can visit Korea with the purpose of tourism without a visa for 30 to 90 days.
Considering international convention, mutuality doctrine, national profit and other such factors, certain counties are granted visa-free permissions. (Listed countries without a specified period have a limit of 30 days)