In the merge buffer, a difference is marked with lines of ‘v’ and ‘^’ characters. Each difference has one of these seven states:
When you select a difference, its state changes from default-A or default-B to plain A or B. Thus, the selected difference never has state default-A or default-B, and these states are never displayed in the mode line.
The command d a chooses default-A as the default state, and d
b chooses default-B. This chosen default applies to all differences
that you have never selected and for which no alternative is preferred.
If you are moving through the merge sequentially, the differences you
haven't selected are those following the selected one. Thus, while
moving sequentially, you can effectively make the A version the default
for some sections of the merge buffer and the B version the default for
others by using d a and d b between sections.
These two states are displayed in the mode line as ‘A*’ and ‘B*’.
Once a difference is in this state, the a and b commands don't do anything to it unless you give them a numeric argument.
The mode line displays this state as ‘comb’.