MIB2 is an industry standard definition (.1.3.6.1.2.whatever...)
Other, proprietary (example, an OS) MIBs are defined by the Company that
makes an SNMP Agent available on their devices. Cisco has a MIB
(.1.3.6.1.4.1.9.{whatever.cisco.wants..), as do Bay Networks, and any
other router manufacturer. They need to register a number; Cisco is 9,
Bay Networks is 18. HP has an Agent on their HP-UX machines. You compile
the MIB definition (generally available at the companies web site) into
whatever Management platform you are using (HP-OpenView, for example).