How much of Partial scope is permitted?

In the context where the "Organisation" is a part of larger organisation, there are few clarifications needed: 1. e.g. a Data centre within an Engg. Organisation. A large no of PCs are connected to the Data Centre. The Data centre hosts all the servers and the applications for an ERP. The application is used by a large no of client PCs located within the same premises or outside on leased lines. (Private network) or may be even on the internet through HTTPS. Scenerio 1: Browser based access on the client PC Scenerio 2: Agent loaded on each PC. Then only you can access the application. Scemerio 3: The IT dept. is responsible for pushing the OS updates, application updates at the client end, Virus updates as well as monitors the various other softwares running on the PCs available in the company. (That is their role is not for DC only but maintenance of all the PCs in the company). Scenerio 4: The larger Engg. company has 3 diffrent deptts. One Which runs the DC; 2nd which provides the connectivity to various usesrs/ group of users within the same premises or acro ss various locations in the country and outside. The levels of such users outside the physical premises of the comany may vary from e.g. to a regional office (with say 50 users each) to a sales office (with say only one or few PCs). 2. While the IT dept. is responsible for the maintenance of complete IT infra. including the DC and the client workstations, they want scope to be restricted to DC only excluding the network (LAN/WAN support). Is it allowed. 3. While the scope is partial is is primarily restriced to IT services, The key decion makers and resource providers are outside the IT Deptt. e.g. CEO of the organisation, Fininancial Heard, HR Head, Security Head, Utilities Head etc. 4.Is it true that ISMS Scope and the Certification Scope may be diffrent that too when the organisaion is part of a larger company. In this case, the role of a certification auditor will be confined to see the ISMS within their scope of certification. Who will the external parties (Outside the scope of certification or outside the scope of ISMS as defined by the organsaion).