If you are doing a debug session on standard SAP coding, you should be aware of a few things: if it is an error you are looking for, check the web first - as you are not likely the first person to encounter the error. And what will you do when you find the error? Fix it ? Apply a SAP modification ? You have to understand where your coding is in the grand scheme of things, before ploughing through SAP's thousands of lines of coding just to find out there is nothing you can do about it. So how can the call stack help ? What is a call stack ?
The call stack can be a great help in positioning standard SAP coding and custom coding, as routines and executable modules are all listed on the call stack, available in the debugger as tab "Desktop 2". The system keeps track of which module was called after which module - for "daily operations". The sytem will need to know what to do when a form routine or even whole program ends. The call stack lists all executable components in a single list, and from this list it should be quite easy to find out which of these modules are custom build.
View the working of the program through the call stack - another wonderful insight on how a program works.