Sheet Metal Bend Allowance
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When sheet metal is bent, the material stretches slightly on the outside of the bend and compresses on the inside, so the flat, unbent blank needed to produce a given bent shape is not simply the sum of the finished leg lengths. Bend allowance is the correction added to get the correct flat pattern length.

Material thickness, bend angle, inside bend radius, and the K-factor (which locates the neutral axis, the layer of material that neither stretches nor compresses, as a fraction of the material thickness from the inside surface).

The K-factor represents where the neutral axis sits within the material's thickness during bending, typically between about 0.33 and 0.5 depending on the metal, its temper, and the bend radius relative to thickness. A wrong K-factor produces a flat pattern that's slightly too long or too short, which shows up as a part that doesn't fit after bending.

A sharper bend radius concentrates more stretching in a smaller area on the outside of the bend, and the material is more likely to thin or crack there, while a gentler radius spreads the deformation over a longer arc, which changes how much extra length the bend consumes.

No, though they're related. Bend allowance is the arc length of the neutral axis through the bend, added into the flat pattern; bend deduction is a different value, subtracted from the sum of the outside leg lengths, used in an alternative method for calculating the same flat pattern length. Mixing the two methods produces incorrect results.