Output of UHD "tx_timed_samples", what does the letter "U" mean?

GNU Radio
1

For UHD, when I run the “tx_timed_samples”, the output will be like:

Sent 1000 samples
UUU
Sent 1000 samples
UUU
Sent 1000 samples
UUU
Sent 1000 samples
UUU

What does those “U” mean?

Thanks,