The phrase "letter designations" refers to using letters to represent or identify specific things or categories. It means assigning letters to different objects, positions, or names as a way to label or organize them. Full definition
The candidate must be viewed apart from the party letter designation next to the name. (rocklandvoice.com)
Fare category: Like booking class, a single - letter designation used by airlines to code the purchase price of a cash ticket and the limitations associated with that fare. (rewardexpert.com)
Although such five - letter designations for latitude and longitude, chosen by air traffic control, do not have traditional semiotic meaning, they serve both memory and communication — much like Jacob's canvases, which are hieroglyphs of presence. (artzine.com)