Approval Type Function

Hi.

Could someone please elaborate what the TM Implem Guide means with "Function = any customized program to search for approvers." when defining Approval Rule.   Does this mean that we can write a custom program and call up that program to derive the approver? 

When I choose "Function" as the approval type, then click the LOV, there are seeded data such as Budget Owner, Object Owner, etc.  Where is the custom program coming in to this setup?

Thanks for any input.

Regards,

Narissa