A "coda" is a piece added at the end of the general material in question as a concluding event, remark or section.
Due to Radio Times cancelling their line of comic stories, Gary Russell's original intended 1997 story Deceptions was derailed, and he had to replace it with the story Coda, abruptly ending his Eighth Doctor storyline.
Due to the original version of the Fourth Doctor story Shada being incomplete and unaired in 1980, the story had multiple attempts at a remake, latest being in 2017 with animation, with Tom Baker returning to his role in a live coda filmed as an extra to the now-finished story.