You will want to use IEnumerable<UiPath.Core.QueueItem>
.
Essentially, you can read that as this is an iterable collection that contains nothing but UiPath.Core.QueueItem
objects.
Beware of Imposters
Be careful when you choose the generic type associated with the IEnumerable
class though. There are many similarly named classes that can cause confusion, and if you choose the wrong one you'll end up with an error about being unable to case between one and the other.

Automatic Type Setting
When in doubt, allow the UiPath Studio tool to set the type for you. Just move the cursor into the input window for the field in question in the Properties tab and type set var: items
. UiPath Studio will then register the variable and set the object type to the appropriate class. This takes the guess work out of figuring out which type to use.