Is there a term used for everthing collectively south of the head or do you describe this as just battery tube and tail? I hope this is correct place to ask this.
To me, it's like with (sub)atomic particles. There's usually a name for the whole and a name for the parts when you subdivide.
Flashlight consists of head + body
head consists of emitter, reflector, lens | glass, polarity protection etc
body consists of battery tube + tailcap
tailcap consists of tail switch, housing and spring