When we should select historical mode in tag item property for historian tag configuration.
This Historical Mode is more suitable for RTU or similar devices where SCADA connection with device is not continuous.
For example ,if you want to log a tag after every 10 sec. then you have to change it`s item property to "Historical Mode" and to acquire tag data for a tag in historical mode the tag must be bound to a trend tag of VJC to allow time-stamped data to be
retrieved from the SCADA source.