Difference between an exit (smod/cmod) and a user exit

What is the difference between an exit and a user exit? When I run the program that is floating around (The one that displays all <b>exits</b> for a tcode) I see many exits but not the <b>user exits</b> that we use. This tells me that there is a difference between the exits in smod and the user exits. For instance, if I run the program for VF01 I get the following:

Exit Name Description

SDVFX007 User exit: Billing plan during transfer to Accounting

SDVFX008 User exit: Processing of transfer structures SD-FI

SDVFX009 Billing doc. processing KIDONO (payment reference number)

SDVFX010 User exit item table for the customer lines

SDVFX011 Userexit for the komkcv- and kompcv-structures

V05I0001 User exits for billing index

V05N0001 User Exits for Printing Billing Docs. using POR Procedure

V60A0001 Customer functions in the billing document

V60P0001 Data provision for additional fields for display in lists

V61A0001 Customer enhancement: Pricing

SDVFX001 User exit header line in delivery to accounting

SDVFX002 User exit for A/R line (transfer to accounting)

SDVFX003 User exit: Cash clearing (transfer to accounting)

SDVFX004 User exit: G/L line (transfer to accounting)

SDVFX005 User exit: Reserves (transfer to accounting)

SDVFX006 User exit: Tax line (transfer to accounting)

However we are not using any of those. What I want to know is when do I use one of those verses using the user exits in the include RV60AFZC. We are currently using USEREXIT_FILL_VBRK_VBRP.

Davis