Mandatory
Within the custom field settings, the checkbox "Allow Empty" determines whether or not the field is treated as mandatory.
New custom fields default to having "Allow Empty" enabled, meaning that they are not mandatory fields.
If a field is mandatory, and the object is saved or submitted with a blank value, the user will be prompted with the message entered into the Validation Message section of the custom field settings.
If the Validation Message is left blank, the text "X can not be empty." will be displayed in a pop-up, where X is the custom field's caption.
The captions of mandatory fields automatically have special formatting applied to them to indicate that the given field is mandatory. You can set a field to appear as a mandatory field by selecting the "Appear Mandatory" setting in the custom field settings. This does not make the field mandatory, but the caption will have the special formatting applied to it. This setting is used in conjunction with the JavaScript Validation function in cases where customized validation is implemented on a field.
