When a colleague can't do their tasks due to out of office reason you can be assigned to work as a substitute performer. This can be great when you don't want the processes to stop and wait for the original performer to return. 

When a colleague enables the Out of office function and add you as a substitute performer you get an email stating the from and to date, a message and the name of the regarded application(s).

How to work as a substitute performer

To access the original performers tasks you go to your lists.

Under your own tasks and lists shared to you there is a new section visible [Original performers] list. There you can access all tasks connected to application(s) the original performer assigned to you. 

What happens when the original performer returns

When the original performer returns, at the specified end time or when the out of office is terminated prior to the end date the substitute performer gets an email with the information that the original performer has returned. 

This also makes the additional list to disappear and the substitute performer will no longer have the ability to access the original performers tasks.