Hi Dilip,
The difference between the .csh and .sh depends on the type of shell your unix machine is using.
You use the .csh file when your unix environment is set to SHELL=path/csh
You use the .sh file when your unix environment is set to SHEL=path/sh
ref: http://www.techmagazinez.com/2013/12/unixlinux-difference-between-sh-csh-ksh.html
When you run the installer the SYBASE.env gets created with the exact location of that instance so the exact path of the created environmental variables is there. So basically the .csh or the .sh file read the SYBASE.env and set the environmental variables if they are not set in a session.
Thanks,
Dawn Kim