Sealable equipment. This has nothing to do with sealing against water ingress. It's a security and safety consideration that requires putting the equipment in an enclosure. The live parts of such equipment can't be accessed without opening the enclosure. With the enclosure open, the equipment isn't operable.
Service. The key concept here is a utility provides the power. A service consists of the conductors and equipment that deliver power from that utility to the premises wiring. A large structure might have multiple services.
Service conductors. The location of these conductors on a wiring diagram is what identifies them as service conductors. Not all service equipment conductors are service conductors — only those conductors that run from the service point to the service disconnect are.
Separately derived system (SDS). To be an SDS, a system must have no direction connection between its circuit conductors and those of another system. Additionally, its power must come from a source other than a service. It's common to switch power sources between, say, the utility and a gas generator, using a transfer switch. The generator is an SDS. The two systems are still connected through grounding and bonding. You don't create an SDS by removing bonding jumpers; you merely create a dangerous system.