Exact meaning of IL offset within a stack trace.

I understand that when an exception is thrown (and there's no PDB around) the stack trace exposes the IL offset of where the exception arose (in the lowest frame).

I'm not very clear though on what this means, is it the IL offset of IL operation that thew the exception or is it the IL Offset of the previously executed IL operation or what?

I'm examining ILDASM dumps of assemblies and the IL offset sometimes doesn't quite make sense.

e.g. one null reference exception reports an offset IL_0063 in the stack frame, but that is just an operation br.s

Any help on exactly how the system determines the IL offset when it reports the stack trace, greatly appreciated.

Thx