The European Standard EN 338 contains a list of strength classes for structural timber. There is a set primarily for softwoods which begin with the letter C (e.g. C16 and C24) and a set for hardwoods which begin with the letter D (e.g.D24 and D30).
Firstly – the 2016 revision of EN 338 allows hardwoods to be assigned to C grades…so they are no longer exclusively for softwood. But there is a more fundamental issue…
Softwoods are conifers. Conifers are gymnosperms – cone bearing seed plants.
Hardwoods (or broadleaves) are angiosperms – flowering plants. More particularly, they are dicotyledons (they have two seed leaves) putting them in a different category from bamboo (which is a monocotyledon, one seed leaf)
Deciduous means that the tree loses its foliage for part of the year – as opposed to evergreen, where the foliage is kept all year.
Hardwood is not synonymous with deciduous. Many tropical timbers are evergreen – so it would certainly not make sense to think that the D in D70 Greenheart stands for deciduous!
And while we are on the topic – conifer is not synonymous with evergreen either! The most familiar deciduous conifers are larches – but there are a few others too (list below).
So what do C and D stand for? Well – they don’t need to stand for anything. But if they had to – Conifer and Dicot would make slightly more sense.
Larches, which include
Larix decidua (European larch) [LADC]
Larix kaempferi (Japanese larch) [LAKM]
Larix x eurolepis (Dunkeld larch) [LAER]
Larix occidentalis (Western larch) [LAOC]
Larix gmelinii (“Siberian” / Dahurian larch) [LAGM]
Pseudolarix amabilis (Golden Larch)
Taxodium distichum (Southern / Bald cypress) [TADS]
Taxodium ascendens (Pond cypress)
Glyptostrobus pensilis (Chinese Swamp Cypress)
Metasequoia glyptostroboides (Dawn Redwood)
[XXXX] is the EN 13556 species code