"To come" is a printing and journalism reference, commonly abbreviated to "TK". The abbreviation is used to mark where additional material will be added to a manuscript before publication.[1][2] It is used without periods.[3]
The use of the abbreviation is to prevent the phrase "to come" from being mistaken as a deliberate part of the text.
Since very few English words feature the letter combination TK, it is more easily searchable than TC. The abbreviation TK and the repeated TKTK are "unique and visually arresting" strings that are both easily seen in running text.[1]