If someone with an account at bank A transfers some funds to someone at bank B, this transfer can occur by bank A decreasing a number and bank B increasing it. That's "just a message". However, it also is only part of the story. The only reason why banks A and B are able to offer this nice service of increasing some number here and decreasing some other number there is that both of them have an account at a central bank, over which they can eventually settle this transfer (together with a lot of other transfers, of course) amongst each other. Otherwise, bank B would suddenly be in debt with bank A, which is a situation that both of them can only tolerate for a very limited amount of time (and money).
That's why I consider the capability to settle the individual transfers to be more important than the capability to negotiate these transfers, and the time of settlement to be the actual time at which wealth is transferred. Before that moment, the transfer is effectively incomplete, which usually doesn't matter much for smaller sums of money, but in case of large amounts that introduce large imbalances between banks you can be pretty sure the banks will want to settle among each other first before the recipient gets full access to the transferred money.
However, all of this is based on the assumption that Swift is used for inter-bank communication as in "non-central-bank to non-central-bank". I always assumed that central banks would have their own communication channels with the non-central-banks and would not have to use some intermediary like Swift, as they themselves are a central entity able to standardize message formats and such.
That's why I consider the capability to settle the individual transfers to be more important than the capability to negotiate these transfers, and the time of settlement to be the actual time at which wealth is transferred. Before that moment, the transfer is effectively incomplete, which usually doesn't matter much for smaller sums of money, but in case of large amounts that introduce large imbalances between banks you can be pretty sure the banks will want to settle among each other first before the recipient gets full access to the transferred money.
However, all of this is based on the assumption that Swift is used for inter-bank communication as in "non-central-bank to non-central-bank". I always assumed that central banks would have their own communication channels with the non-central-banks and would not have to use some intermediary like Swift, as they themselves are a central entity able to standardize message formats and such.