A covered call sells one call per 100 shares. A ratio write sells more, which increases income and introduces genuine upside risk, because the extra contract has no stock behind it. Above the strike the position gains from the shares but loses from the uncovered call, and eventually the loss wins.
Requirements reflect this. The uncovered contract is margined as a naked-call and needs the corresponding option-approval-level. Ratio writing is a legitimate income technique in range-bound names and a disaster in a takeover candidate.
Example: you own 100 XYZ at $50 and sell two 45-day $55 calls at $0.80 each, collecting $160. Break-even to the upside is around $61.60. At $70 the shares gain $2,000 and the two short calls lose $3,000, netting a $840 loss including premium.
Related: covered-call, ratio-spread, naked-call, undefined-risk