From the audience's perspective, the focus of the story's throughline's concern changes from each signpost. For example, Conceiving an idea to developing a plan is a logical progression whereas an event in Act 2 leads to an idea's conception. The signpost is what you're working toward so that when you reach it, the audience (should) feel/understand the change in focus in the throughline onto developing a plan (wherein the focus of the journey results in how the story reaches that particular point.)
There is one signpost per throughline, per act which gives you 16 overall. As for the order in each act, it doesn't matter; it's all how you decide to weave/tell your story. The only hardened rule is you have to fulfill, so to speak, the signposts in each act before moving onto the next act. Again, whether you order OS, IC, MC, RS or IC, MC, RS, OS is totally up to you and however you choose to sequence them to tell your story. Likewise, some may feel more important than others depending on the story you're telling and where your emphasis lies.
As for the relation of each signpost to each other, each throughline is exploring a different concern from a different perspective so the only real relation is perhaps to the OS in the sense of that's why all the characters are there in the first place, but that's really fudging it. It's the end result, the notion that all areas have been explored thoroughly, that's important. This is where the software really makes it easier to understand because when you're going through and answering questions, it works behind the scenes to lay the foundation and build the framework and taking all the guesswork out of it.
You may want to check out this post I did earlier today from a couple of years ago documenting how I worked through all these issues in writing my first script with Dramatica (it was my fifth overall.) It should give you a better understanding as I essentially discuss in some detail exactly some of the questions you're asking: http://discuss.dramatica.com/t/far-end-of-the-black-script-creation/119.