If you are talking about cross sections, a surface will give you a line (two dimensions) in a section view, while an alignment will only give you a point (one dimension). In other words, a cross section is cutting a three-dimensional surface along a line to give you a two-dimensional projection. With an alignment, you're starting with a two-dimensional line, so the section would cut that down to one dimension.
Are you talking about the center point of your assembly? I must be missing something.