A '.fab' file is the native format of 'Cadence Allegro Fabmaster', or of 'FABmaster'.  Both tools export '.fab' files, but with different content and definitions. PCB-Investigator can import both.

Cadance Fabmaster (.CAD) contains component, package and layout information (incudling nets). It can contain silkscreen layer with text and documentation layers with additional information. It also has drill information and some simple properties.

Fabmaster (.fab) contains components, padstacks, testpoints and net information. Thereare different shapes similar to ODB++ available and handles all important layers in simple way. For the import there are different surface options, you can switch between Close the tracks, combine touching elements and try to close them or let everything as it is.