Welded wire reinforcement is the factory-made grid of the slab trades: plain or deformed wires welded into sheets or rolls per ASTM A1064. The style designation decodes completely — in 6×6-W1.4×W1.4, the first pair is the wire spacing in inches each way, and the W-numbers are the plain-wire cross-sectional areas in hundredths of a square inch (W1.4 = 0.014 in² per wire), with a D prefix instead for deformed wire. Wire area times wires per foot gives the steel area per foot of slab width — the number a crack-control check actually uses. Flat sheets are preferred over rolls, which hold their curl and are hard to chair.
What WWR is for matters more than what it is: in a nominal, jointed slab-on-ground its whole job is holding shrinkage cracks tightly closed between joints, not adding load capacity — a welded-wire sheet carries far less steel area than even a modest #3 or #4 bar grid, and where a slab must carry moment across a cracked section the answer is deformed bars sized from drawings. Placement is the quiet decider: ACI 360R wants distributed crack-control reinforcement supported in the upper third of the slab depth, because shrinkage cracks open from the top and light mesh walked down onto the subgrade during the pour controls nothing.