Different Ways to make a field mandatory in salesforce.com

As we know there are different standard ways of making a field mandatory in SFDC. This is a brief about all of those ways. Sure shot CON 201 and ADM 201 Question :-)

• Make a field Required while creating it by checking the Required check box:- This makes that field required for everyone in the organization. In other words this is like making the field required from Field Level security.  
• This will be used if the requirement says " I want to make sure that all sfdc users should enter Salary Amount in the payment details record"

• Make a Field required via Page layout:- This is done to make sure that for that particular Page layout this field is mandatory and for others it is not.
• The Requirement will be like " I want to make salary amount mandatory in payment details object for salary pagelayout but not for Interviewed people in Interviewees salary pagelayout"

• Make a field required by a Validation Rule:- This is to make sure that a field is mandatory on a particular condition.
• The Requirement will be like "Make the Salary Amount mandatory if the  Interview status says Selected"

Certification Question:-  Difference between FLS and Page Layout ( How will you make a field Required and why)

 Note:- Please add if I have missed any other Standard Way :-)