Looking at this switch datasheet, the concepts of "FX port", "FX switch" and "inline FX" crop up. (The obvious Google searches are not really helpful.)
What does FX mean in the context of a network switch?
As Lucas says, FX normally refers to a type of fibre, but that's not how this company is using the term.
Instead, FX refers this company's inclusion of an FPGA that has access to 8 of the ports. People who buy this switch can either buy or develop their own applications that can be loaded on the FPGA and can process data as it passes through the switch.
All 24 ports are 10Gb and they claim the FPGA has 160Gbps of throughput. The idea is that instead of just using the switch to move data and then process the data on the server, you can do whatever data analysis on the fly as it moves through the switch and get faster results.
They use trading systems as an example of where this type of switch could be used, e.g. in the data sheet for their SDK:
FX stands for two strand fibre, it supports up to 100 mb/s and can go up to 2km.