The silver film is an outer jacket to protect against Radio Frequency and ElectroMagnetic Fields.

The only time the outer jacket comes into play is if you are using an RF/EMI backshell (which is rare).

With those backshells the jacket is squeezed between two internal components of the shell. The jacket is thus bonded to the shell and any errant waves are bled off through the shell to ground. This is mainly used on power cables and is used to keep the waves produced by the wires in the cable from going out into the atmosphere and slapping around your low voltage signal.

The drain wire is there to bleed off any induced voltage etc. in your 18/2 to ground. This wire is left open on one end and grounded at the main control panel. If both ends of this wire are grounded you have an antenna. Since these signal cables to not generate enough powerful waves to require backshells the jacket can go away. Some people do not worry about induced problems on cables and don't use either, but I always use the drain.

Unless of course you have a backshell. You can look those up on the websites of Cannon ITT and Amphenol if you want to see what they are like.