Minecraft wiki entry for Structure_Block, associated text: Mirror (¦, <>, ^v)
Sets the mirroring of the structure to none (¦), left to right (< >), or front to back (^ v). At 0° rotation < > mirrors across the X-axis and ^ v mirrors across the Z-axis.
In Bedrock Edition, the options are x and z, which can be toggled to set mirroring.