Consider for example the command:

    echo hello &> world

In both bash and Busybox ash, this command results in the synchronous execution of echo with its output directed to a file named world. However, per the shell grammar, the & should be interpreted as a separator_op, i.e. the complete command is a list, separated by the & operator, where the first simple command echo hi is run in the background and the second simple command is a pure redirection which creates an empty file named world.