In the reference section of Apple's docs there's lots of instances of this sort of thing:
func runAction(_
action
: SKAction!)
The Objective-C 'equivalent' of this is:
- (void)runAction:(SKAction *)
action
It strikes me that it's probably important that (in the Swift reference) there's a space after the underscore and "action" is written in italics.
But I can't figure out what this is trying to convey. So perhaps the question is... is there a reference for the conventions used in the references?
-- here's the page I'm referencing in this reference to the underscore use: https://developer.apple.com/documentation/spritekit/sknode#//apple_ref/occ/instm/SKNode/runAction
Update
Swift 3 has made some changes to how function/method parameter names and argument labels are used and named. This has ramifications on this question and its answer. @Rickster does an amazing job of answering a different question about _underscores in functions that clears much of this up, here: Why do I need underscores in swift?