The Systems Classification Hierarchy (SCH) provides a logical decomposition of the systems, sub-systems and elements that will deliver
an operational railway. The SCH has been structured to identify the core EWR functions (Level 1) from this level the decomposition is
made by identifying the set of systems, sub-systems and elements that would be 'constituents' of the parent function or 'types' of the
parent function. The level of detail a function has been developed to depends on a) the level of maturity at the point the SCH is baselined
or b) the specification approach associated with the system.
The SCH provides the central repository for the system terminologies used across EWR which are further defined in the Functional Information
Requirements (FIRs). It does not mandate the hierarchy or parent-child relationships and simply provides the 'building blocks' for other
structures used within EWR.
The SCH also identifies a number of 'Systems' external to EWR. This is to provide additional context of EWR in the wider railway and
external environment. The purpose of the EWR SCH is not to fully define the complete breakdowns of these external systems, but only aims to
identify the key systems EWR will be interfacing with.