When is the data "acked" to LSF/FileBeat during output progressing

Quick question on the reliable delivery of a log from LSF/FileBeat to a logstash output. At what point is it ACKed. For example, if you are writing to the file output, does it get acked as soon as it it written or only when it is flushed to disk.

Also, what about if you have two outputs? are both required to be successfully for the message to be ACKed or just 1 or ??

Norm