TK

 TK  is an abbreviated editorial mark made when writing, proofreading, or editing to indicate that a portion of the piece is  to come  some time in the future.

Writers often use the combination when writing so as not to slow down the flow of their thought when they might otherwise need to look something up or do some research.

Because the letter combination TK is very rare in the English language it is easy to do a search or search/replace for the mark in digital documents.

Medium
When composing text in Medium if one writes a stand alone TK within the text, the text editor shows a yellow TK within the margin as an indicator to return to that place to finish the thought(s).