Most replies here aren't very useful. At my airline the second officer is a new hire. He serves as a relief pilot, able to occupy either seat. Under normal conditions he can only occupy this seat above 20,000 feet. Under abnormal conditions they are trained up to the point that a seat change can occur. Or in case of incapacitation up to landing as pilot monitoring. Their training and checking reflect these limitations. Usually they do this around 2-3 years, before moving on to the european fleet as first officers. They wear 2 stripes.
I am sure that the function differs per airline, but this is how it works at mine.
Greetings O.