The Restrict per visitor or session condition will let you restrict an Event based on an interval. You will have to specify one parameter in order for the condition to work.
The parameter will let you specify the interval for which the Event should be restricted to.
Selecting session in Restrict per will restrict the Event to only trigger once per session. Selecting visitor will restrict the Event to only trigger once for each visitor.
A session expires when a user closes his browser, while a visitor identifies a unique browser (by cookie) over multiple sessions.
This condition cannot be wired to any Event data.