OEM, Retail, Volume License and bundle/Not For Resale — all is channels of delivery of OS. In a price can figure as — the channel identifier (Channel ID — CID).

OEM — the version is intended for deliveries only with new servers and is unsuitable for update of already existing operating system, supports only pure new installation. This version demands activation within thirty days from the moment of installation;
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Retail — the version — normal «the box version», from the point of view of a customer, the version sold in shops in colourful big boxes. Accordingly, restrictions of an OEM-variant at this version are not present — it supports both pure installation, and update of already existing operating system. Demands activation within thirty days from the moment of the installation;
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Volume License (on the Internet there are abbreviations vl; vlk — volume license key; vlm — volume license media) — that at us «the corporate version» or «the corporate license» often name, this variant is delivered only to very large corporate clients and differs from the normal retail-version that, on pleasure to pirates, at all does not demand any activation.

Actually it not so. At least win 2008 server standard + sp2 demands activation. Probably it is supposed that in a corporate network is KMS server for activation, and if it is not present, alas… Only the activator helps or to look public KMS for the server.
Dmitry Bobrovsky
bundle/Not For Resale — Not for resale.
Dmitry Bobrovsky
Q. Than differ images of distribution kits OEM from Retail?
A. Images OEM and Retail differ from each other only one file — ei.cfg , to be exact one line in this file. In this line the propagation channel is underlined (can take and edit). More no differences in images Retail and OEM are present!

On a disk with Volume License I at all did not find such file.

Запись What difference between OEM, Retail, Volume License, bundle/Not For Resale впервые появилась Dmitry Bobrovsky Blog