In Xcode this is called “Code Sense”. And these icons also exist in Xcode 3. These icons are also used in symbol navigation.
Red: macros
= macro (think #define)
Brown: Core Data / namespace
= modeled class
= modeled method-
= modeled property -
= C++ namespace
Orange: aliased types
= Objective-C category
= enum
= typedef / typealias
= Swift extension
Green: variables
= binding
= function
= field
= constant
= local variable
= IBOutlet
= variable (can be ivar, global var, local var, etc.)
= parameter (think f(x))
Blue: methods
= IBAction
= method
= property
Purple: aggregate types
= class (Objective-C, Swift or C++)
= class extension
= Objective-C/Swift protocol
= struct
= union
Gray: snippets
= snippets
Icons: #pragma mark or comments
= !!!: ...
= FIXME: ...
= ???: ...
= TODO: ...
= MARK: ...
You should be able to look up the meanings from the filenames from /Developer/Library/PrivateFrameworks/DVTFoundation.framework/Resources/Xcode.SourceCodeSymbolKind.*.Icon.tiff*.
In later versions of Xcode, the path is moved to /Applications/Xcode.app/Contents/SharedFrameworks/DVTFoundation.framework/Resources/Assets.car, see Analysing Assets.car file in IOS on how to extract the images from the *.car