What makes someone a 'Senior First Officer'? Some airlines have a Senior First Officer rank. Apart from the title and (presumably) the pay, what makes them different from a regular First Officer?

Do they have any differing duties or responsibilities? And do they need to pass some kind of test, or is the rank just given to them after a certain amount of years service?
&quot;Senior,&quot; is generally a subjective term.  In order to fly to the lowest authorized minimums one generally needs 100 hours in type, but other than that, one is no longer &quot;green,&quot; after accumulating 100 hours in type.