Hasta La Motion!

Im new here, So bare with me:)

I have a question regarding various VFX roles and demo reel advice. I was woundering as a VFX artist do you need to know and show everything in a demo reel, such as modelling, rigging, animating, texturing, lighting, compositing, dynamics etc etc. Or can you do a demo reel specifically just in camera tracking or rotoscoping or dynamics to show what you are good at?.

What I ma basically asking is do I need to know everyhting or can I produce a demo reel just showing 1 thing like dynamics or compositing?

thanks for any replies:).