Before introduction of computers, an additional skill of proofreading and document editing were critical.
The adjustable arm on the copyholder aids legibility and maximizes the typing speed.
There could also be an adjustable ruler and marker to help the typist keep their position when they are interrupted, clips to hold the pages in place, and a light.
The copy can be hand written notes perhaps from an author of a book, a play, or a TV show.
A copy typist or a secretary with this skill will quote their speed in words per minute (abbreviated to wpm) on their curriculum vitae and may be asked to demonstrate their speed and accuracy of this skill as part of the interview or application process.