Flank

The creatures on the far right and far left of a player’s battleline are on the flanks of the line. A creature in this position is referred to as a flank creature. Any time a creature enters play or changes control, the active player chooses which flank of its controller’s battleine it is placed on.

If a battleline only has one creature in it, that creature is on both the left and right flank and is considered a flank creature.

—Official rules v1.8 March 2021

Commentary

If there is a single card in a battleline, it is both in the Center of the Battleline and on both flanks.

Source: Flank•Center of the Battleline