This is STEPcode, formerly the STEP Class Library. It works with EXPRESS (ISO10303-11) schemas and generates SDAI-compliant C++ bindings from the schema. It works with schemas that are part of the STEP standard as well as ones that aren't, such as ISO15926 or IFC.
The generated C++ can read an write a particular type of Part 21 file - i.e. AP214/automotive design, AP227/plant spatial configuration, AP238/STEP-NC, ISO15926, and IFC, to name a few.
STEP files can be written in various formats, including ASCII (Part 21 or P21), XML (P28), etc. SCL does not currently support Part 28.
