When to use () and [] in PKG files?

The () are used for software dependencies, while [] are used for product dependencies. () will work in most cases, but it will throw an error if you state your file supports both UIQ 3 and S60 (which is valid for shared DLLs).

Note that, as the time of this writing, some of the documentation uses
(0x101F6300), 3, 0, 0, {"UIQ30ProductID"}
(0x101F6300), 3, 0, 0, {"Series60ProductID"}
when it should have instead
[0x101F6300], 3, 0, 0, {"UIQ30ProductID"}
[0x101F6300], 3, 0, 0, {"Series60ProductID"}