Swordfish (Size-3 Fish)

A Swordfish is a size-$3$ fish.

When a candidate digit $d$ in three rows or columns (the base set) falls only in three columns or rows (the cover set), then no matter how $d$ is placed, each of the three cover-set houses must contain exactly one digit $d$. Therefore, candidate digit $d$ can be eliminated from all other cells in the cover set.

For example, in the board below, take three rows as the base set. Look at candidate digit $1$ in row $3$, row $6$, and row $9$: it appears only in column $2$, column $4$, and column $5$. So regardless of how these three $1$s are arranged within row $3$, row $6$, and row $9$, column $2$, column $4$, and column $5$ must each contain one digit $1$. Therefore, in those three columns, candidate digit $1$ can be eliminated from every cell except the intersections of the base set and the cover set.

Note that in row $3$ and row $9$, candidate digit $1$ does not appear exactly three times, but that does not prevent them from participating in a size-$3$ fish. It is enough that all occurrences of candidate digit $1$ in the entire base set lie within the cover set.

For another example, in the board below, take three columns as the base set. Look at candidate digit $5$ in column $2$, column $6$, and column $8$: it appears only in row $2$, row $4$, and row $7$. So regardless of how these three $5$s are arranged within column $2$, column $6$, and column $8$, row $2$, row $4$, and row $7$ must each contain one digit $5$. Therefore, in those three rows, candidate digit $5$ can be eliminated from every cell except the intersections of the base set and the cover set.