It's a bit different for wood. In most cases "3 x 2in" should actually be written "ex-3 x 2in" which indicates the sawn sizefrom which material is machined, not the finish size (it will finish at about 70 x 44mma in metric), and there is also the fact that if you specified "3 x 2in exact size" then you would get exactly 3 x 2in material. Generally not too important on carcassing, but imperative that you get the exact size when doing jobs like structural beam end repairs and replacements.
I admit that all this stuff can trip up people outside of the trade - for those of us on the inside we just live with it
Try buying hardwoods….they are through and through cut to 1” 1 1/4” 1 1/2” 2” 3” 4”
and generally cut to lengths somewhere around 1 foot increments.
widths are actually random, but traditionally rounded to the nearest 1” for costing purposes, so a board 7 3/8” wide would be charged at 7” and a board 7 3/4” would be charged at 8” (although there is no convention)
Traditionally sawn softwood can be pretty close to nominal size, so sawn 2” x 1” batten usually is 50mm x 25mm
annoyingly there’s no convention on regularised studwork, 4” x 2” can be 44mm thick, 45mm thick, 47mm thick …..hence why joist hangers are made in a range of sizes.
Ive no idea what CLS is