The format is caster.relative.(o/a)(x/y/z).n,n,n
For example:

• caster.relative.ox.2,0,0
  
  
• caster.relative.ax.2,1,1
  

This is used to return coordinate-related values based on a relative coordinate system.
In this system, the player’s current position is treated as the coordinate origin, and the direction the player is facing defines the positive Z-axis.

• The X-axis represents left and right.
  
  
• The Y-axis represents up and down.
  
  
• The Z-axis represents forward and backward.
  
  
• Expressions starting with o return the offset (difference) of the specified axis between the relative position and the player. For example,
  caster.relative.ox.2,0,0 returns the difference in the X-axis between the player's current position and the position that is 2 blocks to the player's left.
  
  
• Expressions starting with a return the absolute coordinate value of the specified axis at the relative position. For example,
  caster.relative.ax.2,0,0 returns the absolute X coordinate of the position that is 2 blocks to the player's left.