Bottle Cap Reject is another form of iron rejection, sometimes called iron bias or iron mask. Bottle Cap Reject is an iron filter, and works similarly, due to the ferrous and non ferrous composition of (most) bottle caps.
The White's Spextra V3i owner's manual describes it as follows:
"• Bottle Cap Reject – Adjusts how aggressively the Discrimination rejects bottle caps and other unusual alloyed iron that contains both ferrous (iron) and nonferrous (non-iron) mixes. Old Square nails for example. • In any Discrimination search mode, Bottle Cap Reject adjusts the degree the audio breaks up (sputters) regarding iron/steel type target signals. [On the V3i] 20 is the most aggressive (offers the greatest degree of break up on iron/steel. OFF provides the minimum degree of audio break up (least degree of sputter on iron/steel. 1-20 offers increasing degrees of audio break up. Caution must be used as in some ground types higher settings causes all target signals to break up or sputter."
To better understand all types of ferrous rejection, one needs to understand the discrimination scale. The following diagram is from the Minelab Manticore manual, and offers a good visual representation of the VDI scale along with how the ferrous filters work. The center line from left to right, is where good, non ferrous targets would appear... on the right (non ferrous) side of the Legend's Ferrocheck . Iron targets tend to move away from the center line in the diagram. The iron filters are represented by the gray areas on the graph. Raising the filter numbers, and/or bottle cap rejection will move the gray areas closer to the center line, rejecting iron and steel, but the closer you get to the center line, by raising the filters/bc, you risk missing the deeper/on edge good targets that tend to drift further from the center line.
Sorry for the lengthy description, but I hope it helps to give a visual understanding of the filters.