They are exchanged by Condor processes to schedule jobs. They are logged to files for statistical and debugging purposes. They are used to enquire about current state of the system. A classad is a mapping from attribute names to expressions . A classad is thus a form of property list .
Attribute expressions can also be more complicated.
There is a protocol for evaluating an attribute expression of a classad vis a vis another ad. Two classads match if each ad has an attribute requirements that evaluates to true in the context of the other ad. Classad matching is used by the Condor central manager to determine the compatibility of jobs and workstations where they may be run. The classad library is available as a separate stand-alone package for use in applications other than Condor. A high-level overview of the ClassAd language and its use in Condor.