You wonder why a field is filled automatically by the system. How to make it happen? Here are some general settings:
1. In the screen level
Firstly find which screen the field belongs to. Take the field 'process order' in t-code COR6 for example.
Click F1 help on the field. Then click button 'Technical details'. You will find the screen number and field name on the popup.
In t-code SE51, check the field attributes. There is a field 'parameter ID' and 2 indicators 'SET parameter' and 'GET parameter'. They control whether the field can store the field content to an internal parameter or get the content from the parameter.
If you go to t-code COR3 and check the same. On Screen 5110 of program SAPLCOKO, the field 'process order' also refers to the same parameter ID.
Therefore, if the indicators 'SET parameter' and 'GET parameter' are set for field CORUF-AUFNR, after you dispaly a process order in COR3, then navigate to COR6, the process order number will be shown automatically.
2. In program level
Sometimes the system can set or get a parameter id by coding directly. It usually occurs in a PBO module. You can set a breakpoint at 'GET parameter'.
3. In customizing
For some transactions you may be able to control it by customizing settings. As an example you can check the KBA 2031843.
2031843 - Fields 'Material' 'Plant' and 'Group' are proposed automatically from previous input in transaction code CA01/CA02/CA03