Many volumes, interfaces and nodes need different delays before triggering alerts and a single setting per alert does not allow this granularity. I am not alone in having to create multiple near-identical alerts just so that they trigger after different delays with Custom Properties selecting the appropriate alert. This should be an element level property which can be individually adjusted.
I also create Custom Properties for each element for warning and critical levels of disk capacities, interface throughput and CPU loading.
All these properties should be individually assignable for creating alerts triggers and embedding in alert emails, changing colours on maps or tables and for reporting.
