This symbol first turns up in HW with the arrival of the Callahan family’s new Apple IIIc+ computer.1 The Apple logo in question is the early striped version, noticeably without the “bite”.2 To Dairine, initially much more interested in the computer itself than any weirdness in its logo, this difference is initially of no interest. But when she clones the computer and takes off with its portable version on her Ordeal, she slowly begins to catch on to what the apple’s biteless status implies. 
The next mention of the symbol comes in TWD, when Dairine upgrades the Callahans’ household computer. The original machine machine arrived with the software version of the Wizard’s Manual installed, making her one of the first “Ordeal early adopters” and entitling her to membership in the Manual software’s beta group. As such she has been added to the “intensive upgrade” list for the Manual’s accompanying hardware. In TWD, the newly upgraded machine on which she’s testing the Manual beta is plainly a tweaked version of the Power Macintosh G4 Cube. Its logo is the modern, non-striped variety, and biteless like its predecessor. Other routine upgrades are assumed to have taken place since, as Dairine’s father mentions in WH that he’s in favor of them (presumably because they’re free). (Dairine’s “beta version” wallpaper, minus the computerized Manual’s lovely rippling-curtain desktop animation, is here, at 1920×1200 resolution.) 
In the YW universe, the Biteless Apple has become something of a symbol of the wizardly implementations of modern technology, and therefore seemed like a perfect choice for ErrantryWiki’s logo. Whether the symbol’s usage in the books also implies some covert admiration of the intrinsic coolness of the products of the company associated with the Biteful Apple in what passes for the Real World, the Powers That Be have so far declined to say. (HW, TWD, et seq.)
1 This computer is, of course, fictional, and loosely based on the Apple IIc/IIc+.
2 It’s interesting to note in passing this source, which shows the front page of the original manual for the Apple I: the apple is biteless. More info about the evolution of the Apple logo in the so-called Real World, and the logo’s connection with the general concept of knowledge and other archetypes, can be found here.