spagiola,
The ER suffix is applied to twin engined Boeing aircraft that are fitted with the appropriate equipment so they can undertake EROPs flights, engines of proven reliability (0.05 shutdowns/1000hrs for 120 min EROPs and 0.02 shutdowns/1000hrs for 180 min EROPs), 4th source of main bus electrical power etc. etc. It is not used on 4 engined aircraft as it is not relevent. The 777 was ETOPs equiped from day one, so there is no need to differentiate it from other aircraft with an ER suffix. The longer range, higher weight version of the -200 is therefore called -200IGW (increased gross weight) and the stretched version becomes the 300 series of course. Just thought I'd add that before somebody asks the obvious question!.......