What is heimdall PIT file? What is PIT file that is generated by heimdall? Is the PIT format a general format which is used by other software as well? or is a format dedicated to Heimdall?  

If it is general format that could be used by other software, there should be manual or standard description of it, is there any such description?
Heimdall is a open source tool, used to flash firmwares for Samsung devices. Heimdall works some kind of different from Odin.Heimdall flashes individual files where as Odin flashes a package files. PIT file in Heimdall is used to tell your device how to boot.

It is general format and are considered one of the original formats for Macintosh file for compression. (.PIT) are MAC OS files. To open file this on Windows OS use 'Smith Micro StuffIt Deluxe 2010'.