The UCD90xxx PMBus Command Reference contains mostly manufacturer specific commands (see UCD90xxx Sequencer and System Health Controller PMBus Command Reference ).
There are also many standard PMBus commands described in PMBus Specification Rev 1.1 (http://pmbus.org/Specifications/OlderSpecifications).
PMBus Spec Rev. 1.1 is a protocol layer on top of SMBus Spec Ver 2.0. The protocol format such as Read Byte, Write Byte, Read Word, Write Word, and so forth, are defined in the SMBus Spec (http://smbus.org/specs ).
For more FAQ please check the following document.