When designing the data structure for a hotel, you want to know which room a guest has checked in to. It won't be good for business if we don't know who is sleeping where! This is exactly what a reference field does: it creates a link between two records, one pointing to another.
When we examined the URLs earlier, we saw they contained two parts: the table and the sys_id value of the record. These are the two items needed to reference a record. So when you create a reference field, you need to select which table it should point to, which is stored in the dictionary. And the contents of the field will be a 32-character string. Sound familiar? Yep, you will be storing a sys_id in that field.
Reference fields are one of the most important items to understand in ServiceNow. The database sees a string field containing the sys_id value, a foreign key. However, this is meaningless to a person. Therefore, the platform allows you to pick a field that will be displayed. For a person...