Most dictionaries I know of say that 'although' has two related but different usages.
For example, Oxford Living Dictionaries define it as follows:
1 In spite of the fact that; even though.
‘although the sun was shining it wasn't that warm’
‘although small, the room has a spacious feel’
1.1 However; but.
‘he says he has the team shirt, although I've never seen him wear it’
Merriam-Webster's Learner's Dictionary defines it as follows:
1 : despite the fact that : though — used to introduce a fact that makes another fact unusual or surprising
- Although [=while, even though] he was hungry, he could not eat.
2 : but, however, though — used when making a statement that differs from or contrasts with a statement you have just made
I don't believe we've met before, although I must say you do look very familiar.
I think his name is John, although I'm not completely sure about that.
I'd love to have dinner with you, although I can't.
The book had a good, although not great, plot.
Both these dictionaries say 'although' has a second meaning equivalent to 'however' or 'but', as opposed to the first meaning of 'despite the fact that' or 'in spite of the fact that'.
Now, the Oxford dictionary shows that the second meaning is subsumed under the first meaning by numbering it "1.1". Even so, I don't understand why this second meaning has to be added in the first place when you certainly can fully understand its meaning simply by the first meaning of 'in spite of the fact that'.
I don't believe that in its second meaning 'although' is syntactically either a coordinate conjunction such as 'but' or a connective adverb such as 'however'. So I think the use of 'but' or 'however' in the second meaning is purely semantic, and that the although-clause is a subordinate clause whether it's used in the first or second meaning.
Furthermore, is there any reason why all these examples of the second meaning have a comma right before 'although'? Would leaving out the comma change the structure and/or meaning of these examples even slightly?