Starting in Strategy 9.3.1, additional logging is available for Distribution Services. The "Delivery Trace" trace is useful when trying to determine details regarding a specific subscription execution. It can be found in the Strategy Diagnostics and Performance Logging tool as shown below:

Note: This log is not enabled by default. An existing or new "File Log" destination must be selected for this logging to be used.
When a subscription that runs on a schedule is executed or the "Send Now" feature is utilized, a subsequent job is created for Distribution Services. This trace will display details about subscriptions that were triggered and the subscription recipients. The following entries are seen in the trace when a subscription is triggered:
2013-12-13 18:38:58.605-05:00 Enter MSIDeliveryEngine::Delivery(), Job <ID = 32> 2013-12-13 18:38:58.611-05:00 Enter MSIDeliveryEngine::hCheckMobileReportFailure().<Subscription '' (ID = D239E3A342DBF5B6F56059AE3357FD9A), Contact 'Administrator' (ID = 54F3D26011D2896560009A8E67019608) > 2013-12-13 18:38:58.617-05:00 Enter MSIDeliveryEngine::PreTransmissionNotification(). <Subscription '' (ID = D239E3A342DBF5B6F56059AE3357FD9A), Contact 'Administrator' (ID = 54F3D26011D2896560009A8E67019608) > 2013-12-13 18:38:58.619-05:00 Return HR = 0x0 from MSIDeliveryEngine::PreTransmissionNotification().<Subscription '' (ID = D239E3A342DBF5B6F56059AE3357FD9A), Contact 'Administrator' (ID = 54F3D26011D2896560009A8E67019608) > 2013-12-13 18:38:58.621-05:00 Enter MSIDeliveryEngine::DeliveryTransmission().<Subscription '' (ID = D239E3A342DBF5B6F56059AE3357FD9A), Contact 'Administrator' (ID = 54F3D26011D2896560009A8E67019608) > 2013-12-13 18:38:58.624-05:00 Enter MSIDeliveryEngine::hDelivery() wtih iNotification = false. Job <ID = 32> 2013-12-13 18:38:58.655-05:00 Enter MSIDeliveryEngine::hGetDeliveryAddresses() wtih iNotification = false.<Subscription '' (ID = D239E3A342DBF5B6F56059AE3357FD9A), Contact 'Administrator' (ID = 54F3D26011D2896560009A8E67019608) > 2013-12-13 18:38:58.658-05:00 Return S_OK from MSIDeliveryEngine::hGetDeliveryAddresses(). 2013-12-13 18:38:58.857-05:00 Enter GetHTMLDataAndImages() wtih includeData = true iProjID = 0 iDeliveryType = 1 iHTMLDoc = false iReplaceString = 88A0DDFF4821C7795BECFE920D9E2080_ 2013-12-13 18:38:58.858-05:00 Return S_OK from GetHTMLDataAndImages(). 2013-12-13 18:38:59.245-05:00 Return S_OK from MSIDeliveryEngine::hDelivery(), iNotification = false Job <ID = 32>.<Subscription '' (ID = D239E3A342DBF5B6F56059AE3357FD9A), Contact 'Administrator' (ID = 54F3D26011D2896560009A8E67019608) > 2013-12-13 18:38:59.246-05:00 Return S_OK from MSIDeliveryEngine::DeliveryTransmission().<Subscription '' (ID = D239E3A342DBF5B6F56059AE3357FD9A), Contact 'Administrator' (ID = 54F3D26011D2896560009A8E67019608) > 2013-12-13 18:38:59.247-05:00 Enter MSIDeliveryEngine::PostTransmissionNotification().<Subscription '' (ID = D239E3A342DBF5B6F56059AE3357FD9A), Contact 'Administrator' (ID = 54F3D26011D2896560009A8E67019608) > 2013-12-13 18:38:59.248-05:00 Return HR = 0x1 from MSIDeliveryEngine::PostTransmissionNotification().<Subscription '' (ID = D239E3A342DBF5B6F56059AE3357FD9A), Contact 'Administrator' (ID = 54F3D26011D2896560009A8E67019608) > 2013-12-13 18:38:59.249-05:00 Return S_OK from MSIDeliveryEngine::Delivery().<Subscription '' (ID = D239E3A342DBF5B6F56059AE3357FD9A), Contact 'Administrator' (ID = 54F3D26011D2896560009A8E67019608) >