Certain rules require ESPER to maintain subexpressions in memory before deciding to fire them or not. These subexpressions consume memory and if left unchecked may cause the service to go down with memory exhaustion. This parameter is a safety measure that keeps such memory hogging rules under check. If a rule exceeds the specified number of subexpressions, its processing is delayed. The default value is 0, which means this setting is disabled. You must set a value if there are service stability issues.
Unless you are encountering a service stability issue, the value can be left as the default, 0.