Questions about screw and hole unit dimensions

Hi, on the specification of some components, it states that the hole is M2.5. However, when I measured in SW, the hole is 2.05mm in diameter. Are they the same thing? I cannot find such relationship by googling. Also, it seems that M2.5 is in ISO unit but under Tools-Options->Document Properties->Unit->Options, there is no ISO option. What should I do in order for the Hole Wizard to display screws sizes like M2.5, M3, M5?

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