Is this the sort of thing you're looking for? https://books.google.com/books?id=kM4tAAAAIAAJ&printsec=frontcover#v=onepage&q&f=false. Page 19 provides special question-by-question instructions for the registrars. Hopefully this is helpful; if there's anything else you need, feel free to ask.
There were three different versions of the draft registration card used during World War I, each of which had different things in their respective boxes 8 and 9. Which version are you asking about?
Thank you. My grandfather's card is C.
Dear Mr. Bramblett,
Thank you for posting your request on History Hub!
We searched the National Archives Catalog and located the General Files, 1917 - 1919; Office Files, 1917 - 1919; and Historical Files, 1917 - 1919 in the Records of the Selective Service System (World War I) (Record Group 163) that may contain regulations and instructions regarding filling out draft registration cards. For more information about these records, please email the National Archives at College Park - Textual Reference (RDT2) at Archives2reference@nara.gov.
We also located Classification Lists, 1917 - 1918 in Record Group 163 that may have additional information about your grandfather’s interaction with the Selective Service System. The records cover every state within the United States. Each entry provides the registrants' order number, name, race, serial number, classification, physical examination information, date to report for military duty and entrainment to the mobilization camp, and the date he was accepted, rejected, discharged, or reported delinquent. For more information about these records, please contact the National Archives at Atlanta (RE-AT) at firstname.lastname@example.org. When you email them, please attach an image of your grandfather’s draft card, as the card includes information that can help locate your grandfather in these records.
Due to the COVID-19 pandemic and pursuant to guidance received from the Office of Management and Budget (OMB), NARA has adjusted its normal operations to balance the need of completing its mission-critical work while also adhering to the recommended social distancing for the safety of NARA staff. As a result of this re-prioritization of activities, you may experience a delay in receiving an initial acknowledgement as well as a substantive response to your reference request from RDT2 and RE-AT. We apologize for this inconvenience and appreciate your understanding and patience.
In addition, we searched online and located the document How To Answer Questions on Registration Card and Instructions for Registrars in the archival collection “World War I Papers. Military Collection. State Archives of North Carolina” that provides instruction for Card C. To view the item at high resolution, left click on the image of the document. You can zoom in further by either making additional left clicks on the document or using the zoom in and out buttons at the upper left hand corner of the viewer. This document has been digitized and has been made available online by the North Carolina Digital Collections, a cooperative venture between the State Library of North Carolina and the State Archives of North Carolina.
We hope this is helpful. Best of luck with your family research!