While you are writing, it is generally best not to break your stride by stopping to puzzle out or fix some inconsistency or fill in a blank spot or detail. Keep writing so you stay in the groove, but toss in a tag so you can easily find the trouble spot later. When doing a read-through of a completed manuscript, you may want to tag problems rather than stopping to fix them so you can stay in the story. Editors sometimes use tags, along with pages of notes on those tags, to flag areas that need rewriting or revision.
Editorial tags need to be unique so they can be easily searched for when it is time to deal with them. Common tags are TK or TC, which mean “to come” and are used to mark gaps in the text where something else is needed. These are letter combinations that do not occur in words, so they should not provide false hits in your search.
I tend to like <<NOTE>> because it is visually catchy as well as easily searched for. I can amend NOTE with a quick comment on what needs fixing and still be able to find them by searching for “<<NOTE”. Others might be <<EXPLAIN>>, <<SHORTEN>>, <<NEED PHOTO>> or <<UNCLEAR>>. You might think you’d have to start keeping a list of tags you’ve used, but you don’t because you can just search for “<<” and pull them all up.
When doing blog posts or magazine articles, I use this same trick to mark photo placements: <<<MyPhoto140625.jpg 300 Left>>> which gives me the title, width in pixels and the alignment. I learned this from a magazine that accepted text and photos through e-mail but needed to format and assemble the articles for print in their own system.
Tags have a variety of uses in both the writing and initial editing phase of your manuscript.. I hope this helps you out as you write.