sysmain.sdb

Application compatibility database Reference

An .sdb file is a compatibility database on Windows. The binary file contains shim rules that change the behavior of older applications, thus they run on new Windows versions. The system database sysmain.sdb ships with Windows, and the Compatibility Administrator tool makes custom databases. The application compatibility engine applies the matching rules when a program starts.

A file extension is only a part of the file name. It does not give a guarantee about the format or the content of a file. But on Windows, the extension selects the program that opens the file.