Where are the rules about what sources are primary?

Question

Maybe it's just too early in the morning and my brain does not properly connect with the rest of the body, but...

...yes, this is a question about primary sources.

What happened is I was talking with some guys I know and we were talking about different versions of the same things in different books, when I mentioned that usual thing about primary sources taking precedence over later publications.
Someone asked me where these things were written so I opened an errata file or two - PHB and DMG namely - and... those rules I wanted to find aren't there, where I expected them to be, at all.

Quoting from those errata:

When you find a disagreement between two D&D® rules sources, unless an official errata file says otherwise, the primary source is correct.

Ok, this I expected. Primary sources are correct. Right. But that's only half of what I needed to support my claim.
I keep reading on, skimming over the part where it talks about tables and text and then there's the definition of what constitutes a primary source. Sorta.
PHB is primary for something. DMG is primary for other things. MM is primary for some other. But I see nothing about precedence between, say, MM2 (with 3.5e upgrade) and Stormwrack.

What I expected to find was some text on the lines of "a book printed earlier is considered a primary source, unless the one printed later says otherwise". I know something like that exists, because I trust people who have been telling me so for ages, both here on this site and somewhere else. I even thing I saw it once on the books but...

...where?

Answer 1

Primary Sources

The following text is at the beginning of each errata file. (This one from the Player's Handbook errata.)

Errata Rule: Primary Sources

When you find a disagreement between two... rules sources, unless an official errata file says otherwise, the primary source is correct. One example of a primary/secondary source is text taking precedence over a table entry. An individual spell description takes precedence when the short description in the beginning of the spells chapter disagrees.

Another example of primary vs. secondary sources involves book and topic precedence. The Player's Handbook, for example, gives all the rules for playing the game, for playing PC races, and for using base class descriptions. If you find something on one of those topics from the Dungeon Master's Guide or the Monster Manual that disagrees with the Player's Handbook, you should assume the Player's Handbook is the primary source. The Dungeon Master's Guide is the primary source for topics such as magic item descriptions, special material construction rules, and so on. The Monster Manual is the primary source for monster descriptions, templates, and supernatural, extraordinary, and spell-like abilities.

Many read this as saying that other texts are primary sources—for example, the Expanded Psionics Handbook as the primary source for psionics (replacing the earlier edition's Psionics Handbook as the primary source for psionics) and Tome of Battle: The Book of 9 Swords as the primary source for martial adepts and martial disciplines—, but the Errata Rule mentions no texts beyond the Player's Handbook, Dungeon Master's Guide, and Monster Manual. DMs must decide which sources beyond these (if any) are the primary sources for their campaigns.

But, clearly, the three core books take precedence over everything each covers, including the Player's Handbook's exceedingly broad mandate to cover "rules for playing the game" (whatever that means). (Note that the Player's Handbook and Dungeon Master's Guide are given non-exhaustive lists, but the Monster Manual is given what's apparently an exhaustive list, taking precedence only with regards those items. This is weird and probably not a huge source of worry to anyone unless one's going to gamer court.)

Secondary Sources

In one question, the D&D Frequently Asked Questions, Version 3.5 (last updated June 30, 2008) answers not an actual rules question but a publication question. That question is reproduced below:

Both [Complete Arcane] and [Player's Guide to Faerûn] include a feat named Innate Spell, but the prerequisites and uses per day differ. Which version is correct?

Unless stated otherwise, any time that a rule appears in two different sourcebooks (other than the PH, DMG, and MM), the most current sourcebook is considered correct and all previous sources are superseded. A book’s credits page lists its publication date (typically near the bottom of the page).

In this case, [Complete Arcane] (published in November 2004) supersedes [Player's Guide to Faerûn] (published in March 2004), and thus its version of Innate Spell should be considered the official version. (41-2)

I can't find another reference for this oft-quoted meta-rule.

In rare cases, this creates confusion. For example, the armor special ability fearsome that originally appeared in the Magic Item Compendium (Mar. 2007) was superseded by the much better armor special ability of the same name in Drow of the Underdark (May 2007) then was superseded again by the original version in the Magic Item Compendium (July 2013).¹

Warning: There are Internet places where bringing the Main FAQ into a discussion gets one mocked, shouted down, and accused of witchcraft. It is cited here only out of necessity, and I will happily edit this answer to reflect rules provided by a more well-regarded source if one can be found.

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1. Which version of the armor special ability fearsome should be used in the campaign? Whatever the DM decides. This DM allows the version from Drow of the Underdark.

Answer 2

there aren't tertiary sources to consider, so your worry is unnecessary. the core rulebooks are primary sources, and supplements are secondary sources. some rules from supplements become errata'd in and the source it's found in counts as a primary source for that ruling in that case, but generally speaking you won't have much difficulty between supplements because they will likely both be secondary sources.

do situation occur at all in which one supplement counts as a primary source and another supplement counts as a secondary source? sure. but it's pretty rare. the only example I can think of is the introduction of Swift and Immediate Actions to the game. in other words, the source which first introduces a new game mechanic counts as the primary source for that rule over other sources that also make use of that mechanic. Action Points, Immediate Actions, so forthe. if two different sources have two different sets of rules for the same thing, the primary source is which ever source introduced that rule in the first place, unless there was an erratta rule change.