Soil pipe is historically defined as cast iron waste pipe. Waste Pipe is a more generic term and waste pipe does not have a predefined material of construction.
Soil pipe was originally the terminology used for cast iron pipe developed for gravity flow purposes in contrast to pressure systems. One of the first plumbing codes was published in 1881 in Washington, D.C. and it contains the following references to soil pipe installations and specifications:
•Sec. 17. When necessary to lay a soil pipe under a building, such pipe shall be of iron with leaded joints, and shall be so located as to be accessible for inspection. Such pipes shall be kept above ground if practicable, shall not be less than 4" in diameter, and shall extend above the roof of the house; this extension shall be at least 4" in diameter......
Many do not know of the history of cast iron soil pipe and use the term soil pipe in a generic manner to designate all iron drainage piping.
The modern terminology used now for the plumbing system that removes sewage and greywater from a building and vents the gases is a Drain-Waste-Vent (or DWV) system. Cast iron pipe usage for building drainage has been declining for years, but it is still specified in some building codes.
As noted - basically the same - I guess since most of it was in soil - it was called a "soil pipe" Cast iron has been the material of choice for many years - but getting replaced by PVC more and more. Easier, quicker, cheaper!!
