A path to the network
Five conditions about software and routing. Until they hold, no address can be judged, because with no path every address fails identically.
Stage B is where the map stops being about furniture and starts being about software. It contains the conditions that decide whether an address can resolve at all, which is a different question from whether a particular address is the right one. Those two get confused constantly, and the confusion is expensive, because the error you see is the same in both cases.
That is the single most useful idea in this stage. When there is no working path, every onion address in the world produces the same failure. A correct address fails. A wrong address fails. An address that was never real fails. You cannot tell them apart, and every minute spent hunting for a better string while the circuit condition is false is a minute spent on the wrong problem.
What enters this stage
Two arrows arrive from stage A. Machine control feeds the browser condition, because provenance of software is meaningless if somebody else can replace the software afterwards. The clock feeds the circuit condition, because the documents the network hands your client are signed and carry validity windows.
What leaves this stage
Two arrows go forward into stage C. Both of them land on telling a mirror from a copy, which needs a working path and a browser already set to the level you intend. Nothing else in stage B goes anywhere: one browser, one job is terminal on this map, and the two conditions that feed it, the room and the security level, reach the end of their chains there.
The order inside the stage is not a sequence of actions
It is tempting to read B1 to B5 as five things to do in order. They are not. B4 is about when a setting is made relative to when a page renders, which is a claim about ordering rather than an instruction. B3 is about a property of the network you happen to be sitting on, which you did not choose and may not be able to change. B5 is about what a browser already contains before you open it. Only B1 looks like a task, and even that one is really a statement about where a file came from.
Mustard marks the downstream direction throughout this site. In this stage the mustard arrows are short, which is worth knowing: a stage B failure is loud and immediate rather than quiet and delayed.
Why a loud failure is the good kind
Compare stage B with the session stage. A stage B condition that is false stops you at once and stops you visibly. You get nothing, you know you got nothing, and you go looking. A stage D condition that is false often lets you carry on for weeks and then presents the bill in one piece. Given the choice, a failure that blocks you now is cheaper than one that files itself away for later.
The map is arranged to make that distinction visible. Count the arrows out of a condition on the dependency map and you get a rough measure of how much is waiting on it. Read roots and endings for the two extremes: the conditions that hold up everything, and the conditions that hold up nothing.