1. TK as the canonical “to come” placeholder
"TK is a very standard term, see William Safire's usage in this 1996 NY Times article" – bobbiechen
2. Draft‑stage convention, usually marked with a colon for easy searching
"I do this a lot but I use “TK:” with the colon to make it unambiguously grep-able (stands out better visually too)" – natbennett
3. Technical interest in its frequency and possible alternatives
"Could you instead use any two numerical digits? Then you've got a tagging system with up to 100 tags." – sublinear