As I understand it, the idea was for a second aging in a different wood, but that it was up to the individual distiller to choose the second kind of cask for aging. I could very well be wrong on this, though.
From the Talisker site...
"The Distillers Edition is unique: after its usual aging process in American oak casks, the whisky undergoes a second maturation in casks previously holding sherry. The experience is older, sweeter and richer than the regular bottlings."
From Shopper's Vineyard, whom I consider to be a well informed source...
"Like all of the Classic Malts Selections Distiller's Edition, this Talisker has been double-matured in a second cask after the original maturation period (in this case in an Amoroso Sherry cask). The experience is older, sweeter and richer than the regular bottlings. This Edition has a very special bitter-sweet harmony where the peat softens on the palate to a rich, sweet, juicy fruit finish.
The nose is extremely sharp and well focused, with quite wonderful sultana sweetness. Chestnut brown, this is a really full-bodied Talisker, the peat, crisp at first, softens to enormous richness in the mouth, then wafts around the palate with sweet, roasted malt and a heathery dryness.
The finish shows deep cocoa notes, magnificent vanilla and lingering, earthy peat. It is exemplary in balance between sweet and dry, with sweetness always having the slight edge.
Bottled in 2009
Distilled in 1998"