What does 'Primary Author' mean in the abstract submission system?

In the abstract submission system, there is an option to mark the primary author. Does primary author mean the first author or the corresponding author?

In the Dryfta abstract submission system, the 'primary author' refers to the first author listed on the abstract. There can only be one primary author per submission. This is distinct from the 'presenting author,' who is the person presenting the paper at the event. The system also allows the option to designate multiple presenting authors, but only one primary (first) author.