I think HIG means "Human Interface Guidelines" here. Seems to be an Apple thing.
I wish more people would avoid or at least introduce abbreviations that may be unfamiliar to the audience.
I think it was a Xerox thing. But since the 90s, everybody has some since.
Oh, and if you want to read one to learn, the Microsoft ones are better than the Apple's.
Microsoft had one too: WIG!
https://news.ycombinator.com/item?id=22475521