ReadFromTag(szTagName) reads the current value of an existing internal or external tag. A single-tag example is var value = ReadFromTag("Simulator.Channel_1.Device_1.Tag_1");. Supply a tag name, not the value itself. The manual specifies that passing an array of tag names returns an array of corresponding values.
MoveValue(szSrcName, szDestName) copies the source tag value to the destination tag. Both arguments must name existing internal or external tags, for example MoveValue("Setpoint", "Simulator.Channel_1.Device_1.Target");. The manual does not define a MoveValue return-value contract; do not treat an assumed return value as confirmation of a successful device write.