BIP report job status - difference between 'failed' and 'skipped'

Summary:

BIP job status - skipped and failed - what do these mean

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We have a number of BIP report jobs running to send out letters / notifications to employees and line managers (BIP reports) on a daily basis, when checking the BIP report job history sometimes these letters have a status of 'skipped' or 'failed' what do these statuses mean?

We had assumed the skipped meant there was nobody that fitted the criteria that day so the output was skipped (as nothing to send) but the same letter can show 'failed' on other days when nothing has been changed in the set up so no idea why a different status is showing