When planning a consultation with a professional engineer to assist you with your projects it is important that you are able to interpret their proposals. The engineer should be trained to communicate their ideas in both a drawn format and a written format.
Structural Engineers are trained to design with a wide range of materials, including steel, masonry, timber, and concrete. When an engineer specifies a steel beam to be used in a structural system, that element was carefully designed to work in that application. In other words, the steel beam is bespoke.
However, to create individual steel beams for every client would be very cost to the steel fabricators and the client buying the steels. The blue book of steel was developed to list all of the standard sizes for steel beams that the steel companies produced. The reason why steel beams are produced in standard sizes is to help speed up the concept design of Built Structures. The major differences in steel beams are their steel cross sections parameters. The cross sections themselves are standardized.
