ExtRef cannot specify a Poll intended to be implemented as a GetDataSetValues request
Via the ExtRef element an SCT can specify expected inputs to an IED and may state a serviceType indicating a service used to receive the data. In most cases, i.e. GOOSE, Report and SV, the serviceType refers to a specific service defined in 7-2.
But -6 is less precise about serviceType=Poll, "… the client shall / will poll the input data by means of read requests."
Which form the Poll read request takes i.e., GetDataValues or GetDataSetValues cannot be specified using the schema and is left to the respective ICTs to sort out.
But a customer may prefer to use GetDataSetValues to minimize network bandwidth utilization or to minimize burden on the IED since the comparable GetDataValues requests are quite large. While a customer may specify an ExtRef and add a DataSet to the IED for this purpose, the DataSet would appear to be unused and could be removed accidentally.
Extend the ExtRef schema to include a srcDataSet attribute and allow the use of srcLDInst, srcPrefix, srcLNClass and srcLNInst when serviceType=Poll.