There are two things on the front.
An hole with a metal rod in it [perhaps a connection for something]
A red piece of glass with something in it. [stupid guess says it's an charging indicator.]
What are these things?
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When looking for information about something, it always is a good idea to do a quick research in the manual. The 600EX-RT's manual shows on p. 6:
So my guess is that you mean the external metering sensor and the optical transmission wireless sensor, which both sit just above the AF-assist beam emitter (that would be my third guess).