The "DDB" (Design Database) file system. Unlike modern tools that save hundreds of individual files, Protel 99 SE saved everything (schematics, PCBs, libraries) into a single Microsoft Access database file.
Documentation highlights that Protel 99 SE may require specific to function correctly. If new software updates these drivers, it can cause errors, often solved by a full reinstallation of Protel to restore its local copies. Protel 99 Se Download
This IS the Patch for Protel99se SP6 for Win7/8/10. - EEVblog The "DDB" (Design Database) file system