The prompt list is useful only if the list of options is small, does not change often. For example, suppose we have a field named Fiscal month, where the users pick the month’s name (January, February, March) while the system stores its key (1, 2, 3 respectively). The value entered in the Prompt value is what is displayed to the user while the value entered in the Standard value is what the system stores internally.
We use field value rules to populate the list of options. This is applicable if we have a scenario where the list changes more often and requires periodic updates. This option is suited to display a fairly large list since it involves creating a separate rule.