The book of order is primarily procedural. There's a rather nice introductory section, which has good things about the faith, but issues such as whether you have to believe in Christ to be saved are not in the Book of Order. That's not its purpose.
The Constitution has 3 parts. The Book of Order is only one. Another of the parts is the Book of Confessions. There's where doctrine is. JM is correct that the PCUSA takes a different approach to the Confessions than the more conservative Presbyterian churches.
In the PCA, as in the PCUSA around 1900, the Westminster Confession was an actual doctrinal standard. You could not be ordained (and possibly not be a member, though I'm not sure of that) unless you accepted Westminster. Minor disagreements were allowed, but mostly it was an actual standard.
That changed, in a process that lasted from 1903 to around 1967. I use 1903 because a set of "interpretations" were added to the Westminster Confession that essentially disclaimed limited atonement, and possibly others of the 5 points. In the early 20th Cent there was a battle over whether certain traditional beliefs (many of which could be found in Westminster) were essential. The conclusion was the they were not. Some of the ideas involved were (from Wikipedia)
The inspiration of the Bible by the Holy Spirit and the inerrancy of Scripture as a result of this.
The virgin birth of Christ.
The belief that Christ's death was an atonement for sin. [This is Wikipedia's wording. I believe it was actually penal substitution. That Christ atoned for sin is uncontroversial. The issue was over penal substitution as the primary understanding of the atonement.]
The bodily resurrection of Christ.
The historical reality of Christ's miracles.
In a series of battles lasting several decades, it developed that the Church was unwilling to enforce all of those beliefs. Not that everyone abandoned them, but that the Church tolerated dissent on them (at least some of them).
This was codified around 1967 with a new confession (the Confession of 1967), and the creation of the "Book of Confessions." The single confession (Westminster) was replaced by a set, many traditional Reformed, but also the Confession of Barmen and the Confession of 1967. At that point the standard for officers was clarified: they were committed to be "guided by" the confessions, but not to believe every point in them.
Today the Constitution includes the Book of Confessions. Officers vow to be guided by them. In my experience, although new officers are trained, they are not specifically asked to review the confessions and explain anywhere they disagree, as the PCA does with Westminster.
This does indeed, as JM indicates, reflect a very different idea of what it means to be confessional. For the PCUSA confessions are documents produced by the Church to express its current beliefs. It is useful to explain and defend the faith to outsiders, and to train members and officers.
This is quite different from the PCA, and I believe also the PCUSA before 1900, where confessional meant having a confession as a doctrinal standard.
It's a matter of judgement which purpose the authors of the classic Reformed confessions had in mind.
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On the matter of salvation. The PCUSA has for many years (certainly since I was in high school in the 1960s) had many people who believe that non-Christians may be saved. This seems to be the view of recent Popes, and as has been documented above, is also common among evangelicals.
An issue not brought out here, but appearing in the same survey, is that many of our members agreed with a statement that all religions are equally a path to God. While I acknowledge that God may save non-Christians, I most certainly do not consider all religions equal. I have very serious problems with the non-Christians religions I know (though I'd prefer not to lump Judaism in this judgement). Some of them can act as rather dim pointers to God, but I very much fear that some of them (I'm going to be diplomatic by not giving specifics here) leave people worse than if they were agnostics.
One thing we know from surveys is that minor changes in wording can lead to very different results. I'm in a fairly liberal PCUSA Church. I very much doubt that very many of our members truly believe that all religions are equally good ways to God. So I'm not quite sure what was behind the answers to that question, but the issue concerns me.