Build: Unix User and Group ID (or the "s" permissions)
A source package cannot include files that have their "s" flag set to the User or the Group file. These are bits 6000 in your file mode and they are generally represented as s or S instead of x since they affect the execution of executables. The entire Debian system works that way and thus we do not foresee any reason whatsoever that those two flags would ever be needed in a source package.
Note that you need privileges to set these flags so it is not likely that when you create a source package that you would ever end up with those flags set.