My definition of a formal consensus is when all parties involved come to consensus. At a gathering, this would typically be through a council process in which all people attended. I was at one regional where 99.9% of the gathering participants attended council. Given that all the other gatherings I've been to, council happens with a subset of gathering participants. Therefore I believe that when ever consensus is reached may be considered formal, but if it is honored by other gathering participants, who did not participate in the formal council, at the end of the day we end up with a mismatch of formal and informal.
A really great example of a combination of formal and informal is the site consented to by Spring Council. While all or a subset of people at the Spring Council may participate in the consensus process, it becomes informal when thousands of people honor the consensus by showing up at the site selected.
A strictly informal consensus would be something like don't shit in the creek. I don't know that a formal council has ever reached consensus on this issue, but gatherers at large are pretty much in consensus that shitting in the creek has bad health implications for human and non-human creatures and the ecosystem at large. If some person does shit in the creek or suggest that as an option, gatherers at large would generally take issue with this behavior and take the time to explain to the clueless person why this is a bad idea - thereby bringing the clueless person into the consensus.
All this being said, there is a lot of great things to understand about consensus that can be applied to both formal and informal consensus. I've included some links below. Take the time to enhance your understanding of consensus so you can be of greater assistance to the gathering when you are there. These tools will help in so many ways. I find that ever gathering deepens my understanding of what consensus really means and how to achieve it. I also believe that without consensus there is no peace. If we all can't agree on something, then we will fight. I also don't believe there is a state of perfect consensus as we are constantly negotiating and re-negotiating consensus in small and large ways. It is this process that I love so much about the gathering and which teaches me so much about myself and the world at large.
There are many more resources out there, take the time to educate yourself and then bring that knowledge to the gathering, share it and use it.
Good consensus links: