The write operation behaves the same as normal copy-write operations.
D3D12_WRITEBUFFERIMMEDIATE_MODE_MARKER_IN
The write operation is guaranteed to occur after all preceding commands in the command stream have started, including previous WriteBufferImmediate operations.
D3D12_WRITEBUFFERIMMEDIATE_MODE_MARKER_OUT
The write operation is deferred until all previous commands in the command stream have completed through the GPU pipeline, including previous WriteBufferImmediate operations. Write operations that specify D3D12_WRITEBUFFERIMMEDIATE_MODE_MARKER_OUT don’t block subsequent operations from starting. If there are no previous operations in the command stream, then the write operation behaves as if D3D12_WRITEBUFFERIMMEDIATE_MODE_MARKER_IN was specified.