That is — service packs are version-specific. Attempting to apply any 2021 update to ASD 2015 will fail or corrupt the installation.
AutoCAD Structural Detailing 2015 functioned as an extension of AutoCAD, utilizing ObjectARX technology to provide tools specifically for structural drafting. It was designed to bridge the gap between structural analysis (via Autodesk Robot Structural Analysis) and construction documentation. autocad structural detailing 2015 service pack 2 2021