Offset Assemblies are a function of Civil 3D that is commonly overlooked.  Offset Assemblies can be used to attach Subassemblies to different alignments other then the corridor baseline alignment.  This is extremely important when working on projects that have:

1) a construction baseline,

2) are adding a shoulder(s) to an existing road, and

3) much more…

To create an Offset Assembly first you need to place an Assembly.  To do this, go to the Corridor pull-down and click "Create Assembly…".

 

The Assembly marker will look like…

Typically you would now start attaching Subassemblies to the Assembly marker.  But wait, if you do that your Subassemblies will have to follow the same alignment and profile as the corridor baseline.  An alternative is to add an Offset Assembly!

To add an Offset Assembly go back up to the Corridor pull-down and click "Create Assembly Offset…".

Pick the Assembly you just placed and pick a point on the screen for the Offset Assembly.  Now you should end up with:

[NOTE: you can add as many Offset Assemblies as you want.]

From this point you can add Subassemblies to the original Assembly or the Offset Assembly.

Below we are adding Subassemblies to ONLY the Offset Assembly.

Why is this critical to know and use?

If you want a corridor model to follow a construction baseline that is NOT the centerline of the road, an Offset Assembly is a way to do it.  Also, if you want to add a shoulder with a consistent cross slope to an existing road section, but have the corridor model (cross sections) be centered on the road centerline, an Offset Assembly is an easy way to accomplish this task and have everything work correctly when it comes time to plot the cross sections (sample lines).  The assembly would look like:

IMPORTANT NOTE: You can position the Offset Assembly on the screen to have a set horizontal and vertical offset OR you can have the Offset Assembly follow an alignment and profile (KEY!).  Also, notice you can switch the alignment and profile per region (see image below)!