According to the BACnet protocol documentation:

Monitors a property of a referenced object and, when predefined conditions are met, saves ("logs") the value of the property and a timestamp in an internal log buffer for subsequent retrieval. The data may be logged periodically, upon a change of value or when "triggered" by a write to the Trigger property. The Trigger property allows the acquisition of samples to be controlled by network write operations or internal processes. Errors that prevent the acquisition of the data, as well as changes in the status or operation of the logging process itself, are also recorded. Each timestamped buffer entry is called a "log record."

Objects of type trendLog have the following properties:

•ackedTransitions

•alignIntervals

•bufferSize

•clientCovIncrement

•covResubscriptionInterval

•description

•enable

•eventAlgorithmInhibit

•eventAlgorithmInhibitRef

•eventDetectionEnable

•eventEnable

•eventMessageTexts

•eventMessageTextsConfig

•eventState

•eventTimeStamps

•intervalOffset

•lastNotifyRecord

•logBuffer

•logDeviceObjectProperty

•logInterval

•loggingType

•notificationClass

•notificationThreshold

•notifyType

•objectIdentifier

•objectName

•objectType

•profileLocation

•profileName

•propertyList

•recordCount

•recordsSinceNotification

•reliability

•reliabilityEvaluationInhibit

•startTime

•statusFlags

•stopTime

•stopWhenFull

•tags

•totalRecordCount

•trigger