A surface in DesignSpark Mechanical has Zero Thickness
Creation is by...
Create new in a Sketch after exiting a continuous profile
Create new by Pulling a Curve into a surface
Made from existing geometry...
Create by selecting a face or face of a solid body and ctrl C, ctrl V ( copy and paste) - very easy.
Create by 'Detach' - a command by right-clicking on the Solid or on a Face of a Solid, or on a selection in Structure Browser.
Methods and result shown below
Selection of a Face from a Solid
Selection of a Body
Note that along with name and graphic change in the Structure Browser, distinguishing surfaces is easy due to an automatic application of a transparency filter
Other Important Attributes of Faces
Faces maybe combined or subtracted from other faces even though they are of Zero Thickness.
Face edges may be Moved , Pulled or even Filled - some examples are shown below.
Extracted ( or copied ) Faces, especially with numerous or complex edges maybe used ( re used ) as an efficient basis for a new part.