Is the guidelines BuildRoot preferred?

[Patrice Dumas]

Hello,

In the Guidelines, there is 

The preferred value for the BuildRoot tag is
.....

What dose is means exactly? I was under the impression that this was 
a MUST item. But Enrico and Matthias disagree in 
https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=189662
and 
https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=188461

What is the exact status of the BuildRoot tag and if it isn't a must
what are the acceptable values?

--
Pat