KD stands for Kiln Dried.
Douglas Fir most likely
Lumber must be seasoned to prevent it from cracking and warping. After being cut into boards, the lumber is dried (seasoned) in a heated drying shed called a kiln or in the open air.
Seasoned lumber has been dried, usually in a lumber kiln, to roughly 6% to 8% moisture content. Unseasoned lumber may be anything from dripping-wet-fresh-from-the-tree to air dried, depending on how long it's been sitting at the lumberyard and on the local humidity.
In my experience, Kiln dried has been most effective to me for fires. If that is what you are referring to then, go for KILN DRIED.
Kiln dried cedar is cedar that has been dried through a heating process. This is done in a kiln.
G. L. Comstock has written: 'Dimensional changes in kiln-dried softwood lumber after surfacing and during storage' -- subject(s): Lumber, Testing, Drying, Evaluation
This is timber that has been dried in a kiln (large heated structure) instead of being dried outside under cover.
Kiln dried Pine will be much lighter and show no signs of any sap or sap leakage. Sometimes will have a more "whitish" tint to the wood, not as yellow as non kiln dried, but that is not always the case. Some studs will actually be stamped as so. Other than that, youll have to trust the word of the lumber yard or mill worker(s). Always ask 2 or 3 and also ask for the person who knows the most about wood, cant hurt.
You can use a 2x3 or a 2x4, always use a KD(kiln dried)Spruce pine Fir. It's not expensive. Prices vary from lumber yard to lumber yard and season to season, check around. I'v seen them as low as 0.88 to $1.49 for a standard 8' stud(It's accually 91 1/2"
SPF means Spruce, Pine, Fir. Four species, White Spruce (Picea glauca), Engelmann Spruce (Piceaengelmanni), Lodgepole Pine (Pinus contorta), and Alpine Fir (Abieslaciocarpa) comprise the spruce-pine-fir species group. All yield high grade timber with relatively small, sound tight knots. Lumber produced from spruce-pine-fir Species is marketed together as SPF. SPF lumber is a distinctly white wood, with very little color variation between springwood and summerwood. The wood has a bright, clean appearance, ranging in color from white to pale yellow, with a fine straight grain and smooth texture. Uses: Kiln dried SPF lumber is used as a structural framing material in all types of residential, commercial, industrial and agricultural building applications. Kiln dried SPF lumber is also used extensively in the manufacturing of prefabricated housing, trusses and other structural components. In addition to producing high quality structural lumber, with a little preparation, SPF can be made into very appealing, economical solid wood furniture. Products: Structural lumber, trusses, prefabricated housing, exterior decking, preservative treated poles, railway ties, outdoor furniture Technical Specifications: SPF lumber is seasoned uniformly in dry kilns to a moisture content of 19% or less. Kiln drying inhibits natural staining of the wood, improves its strength and stiffness, enhances its appearance and increases its resistance to decay and attack by insects. SPF has a high strength-to-weight ratio and is well known for its outstanding working properties. It takes and holds nails exceptionally well and is easily worked with hand and power tools. It has good gluing, painting and staining properties.
A cubic foot of red oak weighs 44 lbs kiln dried to 6%, appx 61 lbs wet. One cubic foot of wood equals 12 board feet of lumber; 1" x 12" x 12" times 12.