What is baseplate in construction?

It floats. The concrete sub often excludes it as steel work, and steel erectors routinely exclude grouting in their standard scope letters, so it lands on the GC unless someone is pinned down. Check the Division 5 specs, the erector's exclusions, and the concrete scope sheet at bid time, and carry the grout yourself if nobody claims it. It is a small dollar item with an outsized ability to stall column plumbing if missed.

Shop welded baseplates are part of the fabricated piece, so their weight rides in the fabricator's tonnage and price. The exception is loose plates, such as setting or leveling plates shipped separately and placed before erection, which can fall to the erector or even the foundation contractor. Read the shop drawings and the fab quote to confirm which plates ship loose and who sets them.

Look first at the typical base details and the general structural notes, since many sets show one detail that governs all columns. On delegated design work, such as metal buildings, the fabricator sizes the plates and the drawings stay silent until shop drawings arrive. In that case carry an allowance scaled to the column sizes, flag it as an assumption in the bid, and confirm it with an RFI or against the shop drawings.