Aux Alignment option is used when defining custom beam paths.  You need to have civil alignments representing the beam paths.

You must have at least two supportlines.
A beam layout is used to specify the general girder geometry for a slab and girder bridge. This procedure is used for concrete and steel girder bridges.

Procedure:

• On the Home ribbon tab, select the Beam Layout tool in the Superstructure group:

  
  or

  Key in BMGEOM BEAM PLACECORRIDOR

• (Optional) Type a Name for the beam layout in the Place Beam Layout tool settings dialog.
• To specify the layout limits, do the following in the view window:
  • data point on the first supportline to use as the start limit line
  • data point on the second supportline to use as the end limit line
  • data point a third time to accept
  The Beam Layout dialog opens.
• Select the Placement Method: Simple or Continuous.
• Select the spans to which the current layout will apply
• Click on Add and select the first civil alignment that represents the first girder.
• repeat the process to add aux alignments for other girders:

• Set the number of beams according to the layout.
• Set the Spacing for all beams to zero as we are using Aux Alignment
• Change the Spacing Reference to Aux Alignment and select corresponding civil alignment
• Validate the layout and click on Save.