I have a stupid question.....
I bought the Park tool chain wear checker cc-2 and realized that there are 2 measurements : .75% and 1%. Which one should I use and why ?

You can usually ignore wear until this indicator hits 0.75%. At that point get out the ruler and check 2-3 spots on the chain to confirm. Once it hits 1%, the chain is generally shot IME. Try to catch it before then so you don't have to replace the cassette as well.

Oh ok, so if I understand well, 0.75% means "It's time to replace the chain" and 1% means "Its kinda late to replace the chain and you might have to replace the cassette now since you've been lazy" right :-) ?

Some people switch already at 0.5%. 1% is bad. When you check the several places, don't think about average - it is the worse place that matters as starting to damage your drivetrain. You should in particular check the elongation across any removable link. The process of elongation progresses a bit like an avalanche and the removable link may be starting the process.

Thank you for the very valuable information !

Maximum of .6 for me. If I go any farther cogs will skip.
When using the CC-2 just take the slack out, if you squeeze too much the measurement pins will bend.

I have the CC-3.2, which has measurements for 0.5 for 10/11 speed chains and 0.75 for 8/9 speed chains.

Use the chain checker as a back scratcher and measure the chain with a good steel rule.

I use the Park chain wear tool as a quick check now and then. Then if it seems to show wear, I use my steel rule.

and to be sure.

Park should label that tool "for quick checks in a shop environment" or similar. Lots of people here have wasted their money on a tool that is unnecessary and inaccurate.