Here is a previous forum post regarding the cross sections in SS3 that contains information that will answer the questions you have..
communities.bentley.com/.../254855.aspx
Included in the response by Paul Cusack is a link that will take you to alot of good information on the Cross Section tools including a section on Dynamic Cross Sections The tools for MX, InRoads, and GEOPAK Cross Sections are the same, so this video also pertains to the questions you have even though the presenter is using MX. Here is a link to the