The present document establishes guidance in implementation and usage of the functionality described in the STI standard ETSI EN 300 797 [2]. Subsets of the STI standard@ called STI Levels@ are defined in order to make interoperable solutions possible for different suppliers of STI devices. The present document defines the minimum functionality an upstream or downstream entity provides on each level to be considered compliant with that level. This version of the present document is aligned with V2.1.1 of ETSI EN 300 401 [1].