The following was written by Karl Renner, an NCO and Prussian veteran of 1812-1815. It was published in 1829 in his memoirs, which were also intended for "those who want to know what happens in war":
The expression ‘war’ denotes in international law the status between independent nations, in which these pursue their rights with force. The rules of war of civilised nations do not allow at all that prisoners of war are killed or hurt. Their captor are only allowed to take possession of their property. Often the prisoners, especially the officers, are released on parole, not to serve again before they have been formally exchanged, and to present themselves whenever it is demanded. Everybody breaking his parole, when caught, will be treated as a deserter without honour. Spies and marauders (those soldiers who singly or in groups, without orders of their officers, dare to commit violent or hostile acts against the inhabitants) can not claim to be treated as prisoners of war, usually they will be sentenced by death without a long trial. During negotiations in times of war, generally bearers of a flag of truce as well as guards left behind to protect individuals or their property are to be treated as inviolable.
It is my impression that this basically represents what was already generally followed during the Napoleonic Wars. "Prisoner of war" here means someone who has been
accepted as prisoner. If an individual wanted to surrender, it was up to the discretion of the victor to accept this or kill him.