What the rule actually says
The 90/10 rule states that no more than roughly 10% of your contributions — posts and comments — should be self-promotional. The other 90% should be genuine participation with nothing to sell. It is a community norm rather than a hard-coded limit, but moderators and spam filters effectively enforce it.
Why it exists
Reddit’s value is that it is not an ad network. The 90/10 rule protects that: it ensures the people promoting are also the people contributing. An account that only shows up to drop links provides no value to the community, so the community — and the algorithm — treats it as spam.
How to actually hit the ratio
Lead with help. Spend the vast majority of your Reddit time answering questions in your area of expertise, with no link and no pitch. When a thread genuinely calls for your product as part of a complete answer, mention it — disclosed — as one option. Done consistently, your promotional 10% lands with credibility because the 90% earned it.