Pluto TX DDS mode:

You can find information regarding data format by running iio_info.  - https://wiki.analog.com/resources/tools-software/linux-software/libiio/iio_info 

The piece of information you are looking for is this - 

IIO context has 5 devices:
        iio:device3: cf-ad9361-dds-core-lpc (buffer capable)
                6 channels found:
                        voltage0:  (output, index: 0, format: le:S16/16>>0)
                         <...>

                        voltage1:  (output, index: 1, format: le:S16/16>>0)

cf-ad9361-dds-core-lpc - Buffer capable - this is the device that contains the buffer

voltage0:  (output, index: 0, format: le:S16/16>>0) - 
this channel is an output that is part of a buffer. 
It's index 0 in the buffer, 
and the format is 
le - little endian
s - signed
16/16 - 16bits data out of 16 bits storage
>>0 - 0 bits shift

Same goes for voltage 1 - but the index is 1.

Indexing means the channel order in which the device fills the buffers. Here it is fairly obvious that the buffer would contain

voltage0_sample_0, voltage1_sample_0, voltage0_sample_1, voltage1_sample_1, voltage0_sample_2, voltage1_sample_2 .... 

For other devices this is less obvious ..