Hi David,
Both the Acquisition Start Time and relativeInitialX provide the same information: the time in seconds from the trigger to the first sample in the acquired waveform.
The only difference is that the relativeInitialX is an output from several read and fetch VIs (ie, niScope Read Cluster) whereas the Acquisition Start Time is a property in a Property Node which can either be read from or written to. Thus you can set the Acquisition Start Time through the property node.
Hope that helps, have a great day!
Brian Spears
Applications Engineer