Letter codes are used to create new designations or set object categories for objects with a new designation. New for version 19.0 is that you can specify a filter for the upper designation that limits the number of proposed designations. There is also a new description field that is used if there is more than one available alternative when creating a new objects. The description text is also stored on the created objects Denom1 field.
In version 19.0 it is possible to create designations that are only numbers, [none] is used as letter code for that purpose. Designations without counter, with only letters is also possible to create if you set the length number part to 0.