what excactly is this and what is its function and does the 200s engine need a battery

 

The CDI box is, more or less, the "brains" of your electrical system. I'm not about to explain any details because it's wasted space. Either your CDI is bad, or it's not. There is no in between.

To answer your second question,...No,..the 200S will not require a battery. The only ATC200's that require a battery are the 200M,E, and ES.
Both of which have electric start.

KNOWSALOT