Thanks Bink once again ...
ESCROW is a process best described like this:
The development team decides that a feature or set of features in the product is ready for test. They give the targeted phase a name, like BETA 1, BETA 2 or so. They focus on getting the code as stable as possible, target a certain build to be "the one". The targeted build is compiled and ESCROW begins. The build is tested thoroughly 5 – 10 days internally and also released to a few major external developers/partners/BETA-testers. If it survives testing and is found not to contain any major flaws it is declared to have survived ESCROW and released with its targeted phase a name. That is why a released build always is 5 – 10 days "old" when you download it – even though it is brand new. Before it is made available it has to survive ESCROW.
During the development of Windows Server 2003, there has been several ESCROW's. During the last quarter of 2002, several builds went into ESCROW intended as RC2, but they didn't survive the intense testing, hence didn't become RC2. (Many of the builds 366x – 369x were intended for RC2 but didn't make it!)
News source: BinkXP