...
CAUTION: When creating Custom Validations, it is better to have a handful of large Custom Logic than it is to have numerous amounts of short Custom Logics defined for your Organizations Custom Validations. This is because as the number of Custome Validations Custom Validations that trigger on the same form increases, application performance begins to suffer, and can eventually start risking application timeouts. It is also important to ensure the same Custom Logic is not running multiple times on the same records as this also will reduce application performance if a large number of records undergo this same change at once, such as the case with any Validations that might trigger from a Service Desk Action. The performance issue originates from the delay in starting a new validation after one executes. When this delay occurs multiple times during the execution of an extended task, such as completing a large number of Service Desk Actions, this can push the process to take too long and trigger application timeouts.
...
Custom Validation shares a set of common syntax and available functions with the other types of Custom Logic. There are also various built-in functions to assist in querying data in order to do the lookups validations require.
...
When a Custom Validation is triggered during an Import the message will be output into the Imported Records grid as an Error. The message content from the Error will appear in the 'Error Description' column of the grid.
...