What's the difference between *.xe66 and *.out file

To elaborate a little bit on George's answer, *.xe66 is used by newer toolchains for C66x (little-endian mode) ELF executables. A *.out file could be COFF, C64x+, or some other executable format. See also rtsc.eclipse.org/.../ITarget.html and the ti.targets.* packages that are part of XDCtools -- for example, *.?64P would indicate a COFF C64+ little-endian file, *.?e66e would indicate an ELF C66x big-endian file, and so forth.

The advantage of using the newer naming convention is that it concisely specifies the embedded application binary interface (EABI) that the file depends on, while preserving whatever the developer specified as the base file name.