@ludicity (also I can attest the “quietly understood” portion is often explicit.
Depends on which company has more power, but “we’ll give you a deal on our software so you’ll pretend to use it, and you write about it on your blog five times and once in a YouTube video” isn’t uncommon, and in writing as a “marketing agreement”.
If you’ve ever wondered why all the startups use each other, well, this makes them appear to have customers when it’s all VC money)