Personally, I haven't see much difference between the two.
If I review the contract, New Release is defined simply as below, without the caveats above:We would like to remind you of the Publication Type field that was introduced in iTunes Producer 2.2. To avoid availability interruptions on the iBookstore, make sure that all titles are assigned a publication type: New Release, Digital Only, or Other.
A New Release publication is defined in your contract and is bound to specific discounts. A Digital Only publication has no print equivalent or is significantly enhanced from the print edition (for example, a multimedia title). An Other publication does not meet the criteria for New Release or Digital Only (for example, an older backlist title that has a print equivalent).
Based on phrasing, printed books can only be considered "new" or "digital only" in the iBookstore if they are beyond a direct translation of the printed version. Otherwise, they fall into the ambiguous "other" category. By not being "new", publishers lose the opportunity to promote the book as such on the iBookstore.
But more importantly, the relationship to LIST price is spelled out for New Releases in the contract, by country. If you violate this price relationship for New Release listings, you run the risk of your book being pulled.(k) "New Release" means any adult Trade fiction or adult Trade nonfiction book during the seven (7) month period following its first publication, regardless of format (e.g. physical or digital).