Sheds are built from different kinds of steel columns, each with their benefits and disadvantages.

C-Section Columns: Weaker & Cheaper
‘C-Section’ refers to the shape of the steel, which has three sides and forms a C shape.
This form of column is weaker than others, though also results in a cheaper project cost.
Because of the known difficulties that arise from C Section (including load stress, high winds and construction problem), we always suggest avoiding this option.

RHS Columns: Stronger & More Costly. 
RHS (whichs stands for Rectangular Hollow Section) is a fully enclosed steel column with four sides. The is also thicker than C Section, giving it much more structural strength.
Building a shed with RHS means the shed will last longer, will withstand heavier loads and stresses, and can be built onto if you decide to extend.