What does chain worn indicators 75% mean? I've bought a chain wear checker made by BBB, but not sure what the percentage of worn is about?

It has 2 sides, one of them indicates that the chain is worn 75% and I should replace it in the near future. The other side shows, that the chain is 100% used and should replace it as soon as possible.

My detailed question :

What does these percentage mean? What is the equivalent of it in the metric system? 

Will 75% worn state affect my cogwheels also? Can I replace the chain to a new one without changing the drivetrain's other elements? 

I use a single-speed bike. Chain width is 1/8 inch. It consists 94 chain. (400km (250miles) / month)
The .75 is not 75% worn per se.
A chain checker measures chain stretch.
Chain stretch is an indicator or wear-and-tear.
Parts of a drive train wear at different rates. For instance the teeth of the chainset will show wear when they become "shark-finned". But this could be long after the point at which you should replace a worn chain.
If you persist in using a worn chain - though you may not notice anything detrimental - but when the chain eventually gives up - and you put a new chain on - you may find it skipping under load through the gears. This requires replacement of chain-rings / cassettes.
An earlier replacement sought - would have prolonged the life of chain-rings / cassettes.

As for your last question - it is difficult to say - if you ask a manufacturer - they will say no difference. Personally, I think the wider chains lasted longer and are stronger than the narrower 10/11 speed chains of today.

Don't know of any exceptional single-speed chains ... just keep it well oiled and it will last longer.