Text is handled by the device functions to plot text. See chapter 25 Text Internals for more information.
The fz_clip_text and fz_clip_stroke_text functions are used to start a clip. Subsequent operations will be clipped through the areas delimited by these, until a fz_pop_clip is seen. See section 22.5 Clipping and Masking for more details.
Sometimes formats (such as PDF) can send text that has no rendered appearance. We refer to this as ‘ignored’ text. This serves a variety of purposes, the most usual of which is to allow text to be copy/paste out of a document when the actual appearance of that text is given in the form of an image or line art.
For example, a product logo may be rendered using vector graphics, which would ordinarily not have any textual meaning. By including some ignored text, then a user can copy the content out as text, or a text-to-speech engine can correctly enunciate the page contents.
An alternative common example would be where a document has been scanned, and then the text within it has been run through an OCR (Optical Character Recognition) process. The OCR engine would typically include its results as ignored text. The final document would look identical to the scans, but would copy/paste as expected.