In its final form, bcnm is a client network manager: it automatically handles network connections for a client machine. It supports Ethernet and Wi-Fi. IP addresses can be attributed statically or via DHCP.
However, given the existence, and work being done, on ifupdown-ng, it is likely that not much effort will be put into the "network manager" part of bcnm. The package as it exists today is still maintained, because some tools rely on the programs and libraries it already provides: a tool to wait for network interfaces to reach a certain state, and a comfortable C API to control wpa_supplicant.
All these commands exit 111 if they encounter a temporary error, and 100 if they encounter a permanent error - such as a misuse.