I was going through RTH1 and for the most part, the keywords lined up pretty well with their definitions. I just came across #242 though and it has the keyword "world" for 界. Now when I look up its definition in any online dictionary, I generally get "boundary" "classification" "category" and stuff like that. Nothing similar to world at all. I'm also kind of hesitant to change the keyword since he emphasizes in the book that you probably shouldn't since they were all picked with good reason. Anyone wanna elaborate on what the reason is for this character?

Probably so it wouldn't conflict with 境

Heisig has the task of producing unique and memorable keywords for every character. Sometimes when an accurate unique word can't be found, he'll use a keyword based on a common word that contains the character. Since probably about 98% of occurrences of that character are in 世界, it's a fairly fitting keyword.

I think it is certainly not of paramount importance to follow the book without slightest change to it. If you are sure in what you are doing, then don`t be afraid to change the keyword/ or to use multiple keywords/ or to add on-yomi and kun-yomi to the proposed keyword. There are some cases where doing so is definetely better, than relying on proposed keywords (kanji for town and village are the best examples).

Chinese has onyomi and kunyomi? OMG! I've been doing it wrong this whole time! ;-)

As mentioned above, 世界 is the reason for the keyword. It means "world."

Anyway, what I want to say is "don`t be afraid of making changes to the system, if you know what you do", be it RTH or RTK or bigger things in life.

You could use boundary (边界） as a keyword, but then you'd need to change 疆.

But really, keywords don't matter. If it's close to a definition that the hanzi is used in, then that's nice, but even that much is really irrelevant.

and as you've got already "border" which isn't so remote from "boundary", better stick to world in my opinion.
and boundary or classification, are in my opinion to be seen as "world" ("animal world"), etc.

This said, I change several keywords, into things that fits me better, using Pablo to check significations.