based on my research and various checks also with Google Maps and Street View, I have reached the following understanding of the bus hubs for the various bus companies operating in Catania, in the Central Railway Station area:
-) AMT (urban transport of Catania, including Alibus for the airport): these depart in the square immediately in front of the Railway station building
-)AST: these seem to depart from the middle of the circular area, just in the middle of Piazza Papa Giovanni XXIII (same square as the Railway Station, but a little bit farther from the building than the AMT buses)
-)SAIS Trasporti: this seem to depart from the smaller parking lot in front of the ticket offices in via D'Amico (surrounded by via D'amico, viale Africa, via Archimede and viale della Libertà), a little bit further North of the station
-)SAIS Autolinee, Interbus and Etna Trasporti: these seem to depart from a larger parking lot or bus station, slighltly to the West of the station (surrounded by via Archimede, viale della Libertà, corso Martiri della Libertà and via Enrico de Nicola).
Is my understanding correct? Can any of the local experts validate?
Even though these are all quite close together, it's important to know where to go, especially if someone is a bit tight with time for an early departure :)