Compare those 2 sentances:
"The base SL distribution is basically Enterprise Linux, recompiled from source.
-snip-
"Q. What is Scientific Linux?
A. Scientific Linux is in essence, a commercial enterprise linux distribution, recompiled.
and compare it with this statement:
"CentOS conforms fully with the upstream vendors redistribution policy and aims to be 100% binary compatible. (CentOS mainly changes packages to remove upstream vendor branding and artwork)."
For me, "basically" and "in essence" is not as same as "aims to be 100% binary compatible"
If you can not see and understand, then I, or anyone else, can not help you to understand the difference.
Ljubomir