OpenVPN "rwRW" logging

When an openvpn client is configured to have a debugging level of verb 5 or higher, one will find text similar to "rWrWRwRwRwRwrWrWrWRw" in their OpenVPN log, appearing when packets transit the VPN interface.

It's clear the messages are associated with reads and writes through the tunneled interface, but what is indicated by the change in case? E.g.: r vs R and w vs W?

I have searched man openvpn but am without much luck so far.

It is in the man page. It means packets written/read to tun/network:

from --verb:

5 -- Output R and W characters to the console for each packet read and write, uppercase is used for TCP/UDP packets and lowercase is used for TUN/TAP packets.