Each World has a Tech Level. You may not take Skills or Equipment that are above that Level. (The way Skills are labeled, TL4+, for example, means Tech Level 4 and above; TL1-7 means "not below TL1, bit not above TL7".) You may take Equipment that is below the World's Tech Level, but not below its general Era (Ancient, Historical, Modern, Future). So for example, in a TL7 World, you may take Equipment of TLs 7 AND 6, but not 5 which is in another Era (nor 8 because that is above the World's TL).