There's no difference. Just a key pre-nstalled: KMS-client in Volume and Retail in all others, some different text lines in ei.cfg file to force different eula show up during installation, that's all. After installation you can freely change product key on any other and this will change the type of your system without limitations between trial, oem, retail or volume.
Thanks for your reply! Generally I know all the "keys" are "temperary" and ensure 200+ days' "normal" running. Although the time period is very long , what if the expiration day comes ? Would my server be disfunctional or strictly-limited ? If I want to PERMANENTLY use "Server 2008 R2" can I touch some approach for bypassing the "Activation mechanism" as are claimed in internet ?