GNU Radio, One Step at a Time: [Discuss-gnuradio] How is "UU" generated

Another question is how "UUU" gets generated---I still got many of
them? Further, "UUU" makes sense when GNU Radio works with an audio
device; however, I am not sure about its use when GNU Radio sends
non-continuous data to USRP. When this "UUU" happens, will it be
possible that some samples get held somewhere or even get dropped? I
assume not, but like to get your confirmation.

Andrew