The ZCL response bit is what controls whether the ZCL request is getting queued in the Module's buffer to wait for a response. The buffer can hold up to 5 requests, and trying to queue up more will cause the extra requests to be dropped. Space won't open in the queue for up to 15 seconds (i.e. the timeout period).

It's a mistake to use this bit for ZCL commands that don't have a response (like the on/off command). You only really need it for those requests that have a response, like a read attributes. Otherwise, you'll see that the request is sent out, but after the 5th one, you will need to wait until space clears out in the queue (which takes 15-20 seconds).

The recommended setting is to turn it off for requests that don't require a response.