Hi Jason,
The "SEE," "SED," etc., are different branches of the IOS trains available for various platforms, such as the 2960, 3550, 3560, 3750, etc.
12.2(25)SE is the IOS train for 12.2 that is designated for the above mentioned switch platforms. Due to new features and/or hardware support, new branches are created, while others are closed.
The current latest IOS in the 12.(25)SE train is 12.2(25)SEE. Other branches such as the 12.2(25)SED, SEC, SEB, and SEA have been closed.
There's a good whitepaper that goes into greater detail of how this all works:
http://www.cisco.com/en/US/products/sw/iosswrel/ps1828/products_white_paper09186a008018305e.shtml#2
HTH,
Bobby
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