It can be useful when debugging a 4D application to find out which form event caused a method to open or code to run. You can use the Form event command to find out the integer value of the current event and then use the constants list to figure out which event your code is responding to. Here is an ordered list (in numerical order by value of the associated constant) to make that process easier (the list is relevant as of 4D v11 SQL Release 4 / August 2009):

1    On Load
3    On Validate
4    On Clicked
5    On Header
6    On Printing Break
7    On Printing Footer
8    On Display Detail
10   On Outside Call
11   On Activate
12   On Deactivate
13   On Double Clicked
14   On Losing Focus
15   On Getting Focus
16   On Drop
17   On Before Keystroke
18   On Menu Selected
19   On Plug in Area
20   On Data Change
21   On Drag Over
22   On Close Box
23   On Printing Detail
24   On Unload
25   On Open Detail
26   On Close Detail
27   On Timer
28   On After Keystroke
29   On Resize
30   On After Sort
31   On Selection Change
32   On Column Moved
33   On Column Resize
34   On Row Moved
35   On Mouse Enter
36   On Mouse Leave
37   On Mouse Move
38   On Arrow Click
39   On Long Click
40   On Load Record
41   On Before Data Entry
42   On Header Click
43   On Expand
44   On Collapse
45   On After Edit
46   On Begin Drag Over
47   On Begin URL Loading
48   On URL Resource Loading
49   On End URL Loading
50   On URL Loading Error
51   On URL Filtering
52   On Open External Link
53   On Window Opening Denied

All of the constants above are of the type Long Integer. A description for teach constant can be found here: https://www.4d.com/docs/CMU/CMU00388.HTM