You have probably seen something like this many times:
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This is usually found in quick&dirty code samples, to define constants. You can also see it in many older APIs, where various constants were defined this way. This is bad. It works, but it’s lazy.
#define is pre-processor directive and should not be used to define numbers and strings that will then be used as part of the actual code. It’s a hard-coded value that lives in its own memory space, outside of your regular scope.
Use proper, modern stuff that Objective-C offers and Xcode supports very well.