Most hybridizers (raisers) have a personal system for annotating their cultivar seedlings. Typically hybridizers produce many crosses between cultivars or species daffodils. A few years later, when the new crosses bloom, the hybridizer may "select it" and assign a seedling number to the cross. Some years after that, they may name the cultivar, then sell bulbs via catalogs and/or register the new introduction with the Royal Horticultural Society's "International Daffodil Register & Classified List".
For example, if you enter "D2333", you will see a Brian Duncan cultivar which he hybridized, selected, and numbered as a seedling before he finally registered it and named it "Burt Cottage".