Is 'SELinux enforcing' responsible? My HTC android mobile is rooted after unlocking the bootloader, but it is with S-ON. When I write some files(I mean copy and paste) inside my '/system' partition for the purpose of applying a 'Mod', it works , but whenever I make a reboot, those files vanish. Is it due to 'S-ON' or is it due to 'SELinux enforced' ? What for is 'SELinux permissive' required?
It is due to S-ON, so changes made to /system while the phone is running is not permanent, and will be reset upon a reboot of the device.  For information on SELinux in Android, review this link https://source.android.com/security/selinux/.
A brief explanation, taken from the above link:

&quot;With SELinux, Android can better protect and confine system services,
control access to application data and system logs, reduce the effects
of malicious software, and protect users from potential flaws in code
on mobile devices.&quot;
&quot;Permissive mode - in which permission denials are logged but not
enforced.&quot;
&quot;Enforcing mode - in which permissions denials are both logged and
enforced.&quot;