A Set of Specifications is a set of documents of different file versions as laid down in the Technical Specification of Interoperability for Control-Command and Signalling (CCS TSI), i.e. the specifications of many aspects, components, interfaces, etc. concerning ERTMS. Each Set of Specifications includes both an ETCS Baseline and a GSM-R Baseline.
A Baseline is defined by a stable kernel in terms of system functionalities. A new Baseline implies significant changes to the kernel by changing existing ones or adding new features. Baseline evolution should be backward compatible, i.e. onboards compliant with newer Baselines can run on a trackside compliant with an older Baseline.