Handling cyclic and acyclic UART messages
Usually in the PLC communication world terms cyclic means data that are exchanged each scan continuously, for example IO variables, meanwhile acyclic means you send something on demand also you receive the response, usually this means the exchange of parameter data, configuration, diagnosis,..etc. 

What are you looking on particular page of linked spec. is the IO-link protocol on the top of physical protocol. It has nothing to do with UART, it could be any other different hardware protocol below it.