Contact.Number1 or Project.Number).
If the identifier is too complex or if there are more systems than there are number fields, then the Foreign Key system is worth checking out.
It is more complex than the number fields, but it is much more flexible.
Example
We have an ERP system that we want to synchronize with SuperOffice. First, we need to create a foreign key application for the ERP system.App_id= 461name= “erp”
Finally, we add a foreign key entry for the ERP ID for the contact
We can retrieve the SuperOffice contact ID using the ERP ID 1234 like this: