Contact
Start with the guide. It is written to answer your question rather than to sell you the answer, and every page ends with the real limits of that part of the product. If it does not cover it, pick whichever address below fits.
The user guide
One page for each part of the product: what it does and how to use it. Every page ends with a "good to know" section listing the limits and prerequisites that would otherwise turn into support tickets.
Sales, partnerships, anything else
Whether it fits, migrating from something else, or working together. Tell us roughly what your business does and what you're using today.
Support
Anything account-specific. Include your company name and what you were doing at the time, which saves us both a round trip.
Something wrong in the guide
Tell us which page. We maintain the guide against the product itself, so a page that's drifted is a bug we want to hear about.
Already a customer?
Raise it from inside the product instead. A ticket carries your company and the screen you were on, so nobody has to start by asking which account you mean.