deals with text in general, such as processing of text by a web app, but is unsuitable for the more specific text tags (text-formatting, text-analysis, text-to-speech, text-editor, text-message)
There are some specific tags referring to the handling of text (text-formatting, text-analysis, text-to-speech, text-editor, text-message), but there are some cases where none of these applies but the handling of text by a web app is still a focus of a question. In these cases the text tag should be used.
Note that more specific tags having to do with text may be created, and any of these applies to a post, it should be used instead of this more general tag.