Each artifact can get its own directory here - If you depend on an artifact that installs assets for its own usage, the dependent artifact will also get these assets installed. Therefore it is important to know, that the directory part of the original artifact will be removed. An example:

You have a module names foo and it has assets in data/foo. You also have a module names bar which depends on foo.

bar will now automatically get the files installed that foo offers. But - not in $install_prefix/bar/foo/myasset.ext - but in $install_prefix/bar/myasset.ext. This is exactly due to the reason that you had to specify the FILES for the engine_add_executable with the full path to foo/myasset.ext.