While debugging an application with IDA Pro and WinDbg, I came across following statement.
mov esi, ds:(dword_16C6 - 16B2h)[esi]
To simplify, let's say
mov esi, ds:(ABC)[esi]
What does (ABC) mean here?
In Intel syntax, round parenthesis are for grouping, just like in mathematics. This, is done because
ds:dword_16C6 - 16B2h[esi]
might be interpreted as
(ds:dword_16C6) - (16B2h[esi])
which doesn't make any sense.
In AT&T syntax, square brackets are used for grouping instead.
In other words, it's a complex way of writing ds:[esi + dword_16C6 - 16B2h], or ds:dword_16C6[esi - 16B2h] which are both also valid MASM syntax (I think).
Some might say unnecessarily complex, but it does give consistency with symbol[index] syntax which this disassembler uses for symbol + register(s) addressing modes.