Cable sizing is the inverse problem of voltage drop : instead of computing the drop for a given cable section, we determine the minimum cross-section that keeps the drop below a target (typically 3% for lighting, 5% for general loads, per IEC 60364-5-52).

The result is rounded UP to the next standard cable section so you can pick the smallest commercially available conductor that meets the requirement. The headroom indicates how much margin the standard section provides over the minimum calculated.