Is there any classification for a specific operating system used for a specific purpose ?
Yes, although SCADA has not quite arrived yet.
Do-178B is a standard used for qualification of system for aerospace. The FAA requires DO-178B compliance for software and systems on commercial aircraft. It is also typically applied to government and commercial satellites. There are operating systems which are DO-178B compliant: GreenHills Integrity-178B RTOS, Wind River VxWorks Cert Platform, HeartOS DO-178B, SYSGO PikeOS, DDC-I Deos, LynuxWorks LynxOS-178.
Theres is also the European Cooperation for Space Standardisation (ECSS). Although ECSS focus more on the process rather than the product.
NIST has a publication Guide to Industrial Control Systems (ICS) Security which says that Industrial Control Systems use "Differing and possibly proprietary operating systems, often without security capabilities built in"
Here are some highlights of vulnerabilities:
"Some ICS operating systems make setting secure passwords difficult, as the password size is very small and the system allows only group passwords at each level of access, not individual passwords."
"Many ICS protocols transmit messages in clear text across the transmission media, making them susceptible to eavesdropping by adversaries."
"Many ICS protocols have no authentication at any level. Without authentication, there is the potential to replay, modify, or spoof data or to spoof devices such as sensors and user identities."
"SCADA and industrial protocols, such as MODBUS/TCP, EtherNet/IP, and DNP318, are critical for communications to most control devices. Unfortunately, these protocols were designed without security built in and do not typically require any authentication to remotely execute commands on a control device."