The PTP 650 AUX port is a gigabit Ethernet RJ-45 port. It can be used for Local Management, Out-of-Band management or for payload data. With the dual-data path feature this port can also carry data in parallel with traffic on either the Main port or the SFP fiber port. Using dual data path you can carry two streams of payload traffic over the link.
Note that there are 3 ports on a PTP 650: Main, Aux, and SFP. The configuration GUI allows configuration of which port will carry the primary payload traffic,which will carry management traffic and whether dual-data payload is configured.
Finally, the AUX port is also an 802.3at POE output port which can be used to power external devices when the PTP 650 is fitted with the AC+DC power injector from Cambium.
Not sure what your application is for an Ethernet converter but it should work if standards compliant.