What this param in chainparams.cpp do?

Question

consensus.nRuleChangeActivationThreshold = 6; // 75% of 6
consensus.nMinerConfirmationWindow = 8; // nPowTargetTimespan / nPowTargetSpacing * 4

1. For what this rule exit ? How to calculate this rule for find valid
2. value for altcoin.
3. What can be happen if it's value to low or to high ?

Is this value attached to the nPowTargetTimespan / nPowTargetSpacing * 4

Or is it completely unrelated to them?

Answer 1

Those parameters are for BIP 9 soft fork deployments. For a BIP 9 soft fork with these deployment parameters, the fork will transition from the STARTED state to LOCKED_IN when 6 blocks (nRuleChangeActivationThreshold) out of an 8 block non-overlapping window (nMinerCOnfirmationWindow) signal for that soft fork.

These values are not necessary for normal operation. They are only used for soft forks, so unless you are deploying soft forks, these values don't matter at all.

Is this value attached to the nPowTargetTimespan / nPowTargetSpacing * 4

No, it is not. There is no relation between these parameters at all.