why should I care about EXTENSION comments in boost documentation

Question

By looking at boost documentation some of the classes and functions are commented as "EXTENSION"

for example like this one, what does this "EXTENSION" mean and why should I care about it?

the comment is supposed to give some notice, but what does it mean for casual learners such as myself?

Answer 1

By looking at what is marked as EXTENSION, I can say that they are entities in boost which are not in the corresponding part of std. boost::thread and related can basically be used as a drop-in replacement for std::thread; the EXTENSION comments seem to mark additionaly bits which don't have the std equivalent.