IEC 61158-3-19:2019 defines the services provided to the Type 19 fieldbus application layer at the boundary between the application and data-link layers of the fieldbus reference model and systems management at the boundary between the data-link layer and systems management of the fieldbus reference model. This fourth edition cancels and replaces the third edition published in 2014. It constitutes a technical revision. The main changes are: -improving the hotplug and redundancy features; -improving the phase switching and the error handling; -editorial improvements