Machines on your line already speak PROFINET or EtherNet/IP, and a 0.75 kW auxiliary axis should join that network as a standard IO device rather than spawning a separate control cabinet. The 6SL3520-2XE00-7AF0 mounts on the machine frame (IP66, 480 V) and drops onto the network without a gateway.

When does it belong on PROFINET instead of running local I/O?

It belongs on PROFINET when you want to commission, diagnose, or re-parameterize the axis from the engineering station, or when the PLC expects it as a network device rather than hardwired I/O. You pay for that with a node to license and a network to keep healthy. If the axis is a standalone conveyor zone that only needs start/stop and a safe stop, the 7AB0 local-I/O variant is leaner — take the 7AF0 because centralized control earns its place.