Halliday & Matthiessen (2014: 696, 698):
The general difference in meaning between the subjective and the objective orientation can be seen from the effect of the tag. Compare the following two clauses:he couldn’t have meant that, could he?
surely he didn’t mean that, did he?
In the first, the speaker wants the listener to confirm his estimate of the probabilities: ‘I think it unlikely; do you share my opinion?’. In the second, he wants the listener to provide the answer: ‘I think it unlikely, but is it in fact the case?’. It is possible to switch from a subjectively modalised clause to a non-modalised tag, as in this exchange in a store selling children’s books:What do you reckon would be good for a five-year-old kid?
– She’ll like fairy tales, does she?
Here the salesperson’s reply means ‘I think it likely she likes fairy tales; is that the case?’ – whereas she’ll like fairy tales, will she? would have meant ‘do you agree that it is likely?’. The speaker is assuming, in other words, that the customer knows the preferences of the child; there would be no point in simply exchanging opinions on the subject.