[ryanworl]

Can you explain how this is useful? It seems like the destination is the important thing here not the source.

[ithkuil]

it can be useful for some coordination problems.

for example imagine that you have N writers and you want only the first of them to write something in a n object.

each writer writes their content into "_foo" and each tries to copy that to "foo", with "x-amz-copy-source-if-match". Only one of them will succeed and "foo" will have one consistent value, which all observers (including other writers) can agree who has won.