Assembly options determine how walls, beams, and columns will be assembled when using the Assemble Part tool by defining the placement and direction of the assembly origin axes. Each part type has its own unique settings.
...
The datum of the assembly origin defines where it is placed along the width, length and thickness of the part.
Width Datum
Beams, Columns and Walls - Set as Left or Right side of the part.
Length Datum
Beams - Set as Front or Back side of the beam.
Columns and Walls - Set as Bottom or Top of the part.
Thickness Datum
Beams - Set as Bottom or Top of the beam.
Columns and Walls - Set as Front or Back of the part.
Length Axis
Axis - Set as the Red, Green or Blue axis as shown in the family editor.
Direction
Beams - Set as “Back to Front” or “Front to Back” of the beam.
Columns and Walls - Set as “Top to Bottom” or “Bottom to Top” of the part.
Thickness Axis