Currently it works like this
1) The Linework / Alignments / Text etc. is always loaded in entirety, however it is layered and you can turn Layers on and off to reduce what linework is visible on screen - so you can use the Layer Controls in Siteworks to turn Layers On or Off as needed
2) Surfaces are listed and you can pick whichever one you want to work with as the elevation reference in Siteworks, that one is then extracted from the VCL and displayed in the graphics screen and used as the reference.
3) Corridors are loaded and selected as a Surface Model
Alan