Sophie Vershbow’s article In Praise of TK: Why the Handy Shorthand Has a Surprising Emotional Hold on Me introduced me to a convention of writing “TK” next to something that isn’t yet finished or polished:
Simply put, TK is publishing and journalism lingo for “to come.” Benjamin Dreyer, copy chief of Random House and author of the delightful grammar book Dreyer’s English: An Utterly Correct Guide to Clarity and Style, explains that TK is “inserted into text, most often by the author, to call attention to something that isn’t yet included at either the manuscript or page proofs stage that needs to be included before a book goes to print.”
As a developer, I’m used to marking parts of incomplete code with a @TODO mark that many editor software highlights differently from the comment and it makes it easy to find all unfinished bits from the work.
Wikipedia article on the topic
“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. It is used without periods.