It's about industry/context. I can start McDonald's Plumbing, but not McDonald's Hot Dogs. The issue becomes one of "consumer confusion." In the plumbing example no one is likely to think my business is affiliated with the burger chain, but in the hot dog example they might.
Apple was allowed to use the name even though Apple Records (the Beatles) had the mark because it was ruled that music and computers were very different domains.....then years later iPod/iTunes were released and the legal wrangling began.
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