Answer
Structure of Management Information Version (SMI) is a subset of Abstract Syntax Notation One (ASN.1), which describes the structure of objects. SMI is the notation syntax, or “grammar”, that is the standard for writing MIBs.
Junos OS supports SMIv1 for SNMPv1 MIBs, and SMIv2 for SNMPv2c and enterprise MIBs.
For a list of supported standard and enterprise-specific MIBs, click SNMP MIBs and Traps Reference