Every bid lead on this site is tagged against these CSI MasterFormat sections, based on the actual project description text - not a generic keyword match. Here's what each one means.
The primary steel skeleton of a building - columns, beams, girders, and the connections between them. If a project is tagged with this section, it needs a structural steel fabricator/erector at the core of the job, not just an ancillary trade.
Open-web steel joists and joist girders - the lighter-weight, long-span framing typically used to support floor and roof decks between the primary structural steel. Common on warehouses, schools, and other large-footprint buildings.
Corrugated steel floor and roof deck, usually installed directly on top of steel joists or beams (often as a base for a concrete topping slab). A project tagged here has real deck-supply/installation scope, even if it isn't a full structural-steel job.
Miscellaneous steel and metal work that doesn't fall under structural framing - stairs, railings, ladders, gratings, and other custom fabricated metal items. Smaller in scope than 05 12 00, but real, recurring work for a fabrication shop.