Why does RegisterUnitEvent responds to all units if the unit parameter is set to nil?

[Lyak]

How's everyone going?!

So, the title says pretty much all.

Why does RegisterUnitEvent responds to all units if the unit parameter is set to nil?

I currently have the following code.

if info.event then
    for event, unit in pairs(info.event) do
        if type(unit) == "table" then
            object:RegisterUnitEvent(event, unit[1], unit[2]);
        else
            object:RegisterEvent(event);
        end
    end
 
    object:SetScript("OnEvent", function(self, event, ...)
        prototype[event](self, info, ...);
    end);
end

This will make an object respond to all units if unit[2] is nil.

Why...?

I could just make an extra if statement to check if unit[2] exists, but before that I really want to know what's the difference between leaving the second unit parameter empty and assigning nil?