I found another article that explains it pretty well:
Explained: What is the ‘Ct value’ in a Covid-19 test?
"What is the significance of the ICMR threshold of 35?
Globally, the accepted cut-off for Ct value for Covid-19 ranges between 35 and 40, depending on instructions from the respective manufacturers of testing equipment. The ICMR has arrived at the Ct value of 35 based on laboratory experiences and inputs taken from several virology labs."