A display business rule runs when a record is loaded before any client-side scripts are run. This type of business rule is useful for providing values to the client side so that there is no need to call the server from a client script. We can avoid AJAX calls using this type of business rule.
Display business rules were not always available in ServiceNow, so you may find instances where using them in old instances can reduce server calls in client scripts and therefore reduce record-loading times.
If you are considering using an AJAX call in an onLoad client script, then you should instead, where possible, use a display business rule to fetch the server-side values you require and set them in a scratchpad for client-side scripts ...
