You're already answering your own question:
U0 stands for the control channel on qubit 0, as you can read in Table 1 of https://iopscience.iop.org/article/10.1088/2058-9565/aba404/pdf.
In your schedule the CX is implemented as echoed cross resonance gate (ECR),
where we drive the control qubit 0 at the frequency of the target qubit 1. Therefore you can see a cross resonance (CR) pulse on the control channel of qubit 0,
U0. Since we echo the pulse, you can also see a secondary CR pulse with inverted angle, interleaved with X gates.
You can also see something happening on the drive channel of qubit 1,
D1, during the execution of the CR pulse on the control qubit. This is a cancellation tone to improve the CX fidelity. Note that even though the channels are drawn separately, they can be implemented on the same wire on the device.