The three Read Status attributes/properties are useful for observing the progress of your acquisition.
The Current Read Position is the place in the buffer where the next read begins if the Relative To attribute/property is Current Read Position and the Offset is 0. In any case, the Current Read Position is always where the last read left it.
Total Samples per Channel Acquired is the total number of samples per channel acquired by the device and transferred into the buffer.
Available Samples per Channel is computed by first calculating the Current Read Position based on the settings of the Relative To and Offset attributes/properties and then subtracting this number from Total Samples per Channel Acquired.