Contrast and concessive linkers

Contrast clause or concession clause ? 

Look at these two example sentences adapted from a grammar book:

• Although everyone was tired, they kept going until it got dark.
  (= concession - their action is slightly surprising given their tiredness)
• Although they accepted some of his recommendations, they rejected others.
  (= contrast - between accepting some recommendation and rejecting others)

Some books for learners would call the first clause in both of these sentences concession clauses. Others would call them both contrast clauses. This particular book calls the first one a concession clause and the second a contrast clause, but not many books make that distinction.

There is not a big difference between contrast and concession, and a lot of the examples we'll be looking at express both concession and contrast to varying degrees. It's an area where even linguists have problems: in one academic paper, the writer calls concession a 'fuzzy' (not clear, confused) concept, so it's not worth getting too worried about the difference.

http://random-idea-english.blogspot.com.es/2014/08/exploring-concession-and-contrast.html