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

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,

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