Why chip load varies by wood type
Chip load is the thickness of material each flute removes on a single pass, and it is the single biggest factor in whether a cut comes out clean or burnt. Too small a chip load means the flute rubs and generates heat instead of cutting cleanly, scorching softwoods and dulling tools fast on denser material. Too large a chip load overloads the flute and tears the wood instead of shearing it.
How the numbers are calculated
Recommended chip load starts from a material-based default for the wood type selected, whether softwood, hardwood, MDF, or plywood, since each behaves differently under a cutter. Resulting feed rate then equals spindle RPM multiplied by the number of flutes, multiplied by that chip load value, the same relationship used across every feeds and speeds calculation regardless of material.
Practical tips
Treat the chip load value as a starting point rather than a fixed answer, since bit sharpness, dust collection, and the specific hardness of the actual board in front of you all shift the ideal number in practice. Listen and look at the chips coming off the cut: fine dust instead of visible chips is usually a sign the chip load is too low, while large, ragged chips or visible tearout suggest it is too high for that material and bit combination.