BS EN IEC 61158-6-21:2019 Industrial communication networks. Fieldbus specifications Application layer protocol specification. Type 21 elements

Automatic control systems
Real-time systems
Data transfer
Local area networks
Data processing
Data transmission
Data circuit-terminating equipment
Industrial
Process control
CSMA/CD networks
Computer networks

Link: BS EN IEC 61158-6-21:2019 Industrial communication networks. Fieldbus specifications Application layer protocol specification. Type 21 elements
Source: BSI Standards