mule - Explanation and example for use of ackExpression in Until Successful Scope -

i trying understand use of until successful scope consfused use of ackexpression attribute, documentation has precious little on here

specifies expression that, when evaluated true, determines synchronous response of until-successful.

the blog here has couple of lines on it, have not been able make sense of it, appreciate example , usecase attribute.

until-successful default operation mode asynchronous. ackexpression allows synchronously produce response payload inbound event.