Vertical spread
Buying and selling options of one type and expiry at different strikes.
How to read the number
It caps both the loss and the gain: selling the far strike returns part of the premium paid, and the far upside leaves with it.
When the metric lies
The legs live towards expiry at different speeds, and the liquidity of the far strike stands in the way of an early exit at the theoretical price, so unwinding happens piecemeal and worse.
Also known as: bull spread, bear spread