What does Putin's High Combat Alert of nuclear forces mean?

On 27 February, in response to what he called "aggressive statements by NATO leaders," Putin told his Defence Minister, Sergei Shougu and the Chief of the Military General Staff, Valery Gerasimov to put nuclear forces in a "special regime of combat duty."

White House called it an "escalatory and totally unnecessary step."

How will Putin's action impact the ongoing conflict? To understand 'practical meanings' of Putin's order, number of questions come to the fore:

Can the conflict boil over to nuclear war, by design or by default?

Can there be fatal miscalculation(s) leading to nuclear conflict?

Is it a preliminary command or it is the highest level of readiness?

Will the order make a retaliatory strike possible or does it mean preparation for the first strike?

Is it a message for Ukraine that limited nuclear strike can be used against Ukraine to maintain the momentum of offensive?

US Defense officials have not disclosed their current nuclear posture. Is it a potentially dangerous scenario?