When you use a dialog editor and insert new controls, they typically are assigned control IDs starting at around 100. Why?

Because the small numbers are already taken.

/*
 * Dialog Box Command IDs
 */
#define IDOK                1
#define IDCANCEL            2
#define IDABORT             3
#define IDRETRY             4
#define IDIGNORE            5
#define IDYES               6
#define IDNO                7
#define IDCLOSE             8
#define IDHELP              9
#define IDTRYAGAIN         10
#define IDCONTINUE         11

The dialog manager knows about these special values and assumes that if your dialog box has a control whose ID matches one of these special values, then it also behaves in a certain way.

The dialog manager assumes that a control whose ID is IDOK is an OK button. If the user hits Enter, the default button will be pushed; if no default button can be found, then the OK button is pushed. Similarly, a control whose ID is IDCANCEL is assumed to be a Cancel button. If the user hits ESC or clicks the X button in the corner, then the Cancel button is pushed.

If your dialog box has OK and Cancel buttons, make sure to give them the IDOK and IDCANCEL control IDs so that they act like proper OK and Cancel buttons. Conversely, any control with those IDs had better be proper OK and Cancel buttons.