Is 'scusi' only used in a formal way? and 'scusa' in an informal way?

Question

Is 'scusi' only used in a formal way? and  'scusa' in an informal way?

Answer

Very easy rule:
Use /scusa/ when you are in 'TU' terms with the person.
Use /scusi/ when you are in 'LEI' terms with the person.

Answer

"Scusa" is used in informal occasions like speaking with relatives or friends.
"Scusi" is used in formal occasions or when you don't know someone .