Well, in Law, Laws by Decree represent the action of the government legislating without needing to consult another body. It was what the Kings did as of old. They wanted something changed and issued a decree to change the law to their wish. Thus the government decrees a Law, and it is wholly passed without needing the approval of anyone else. I have no idea what HMS stands for so bleh. Parliamentarism refers to the political action of issueing and legislating laws whereby most of these need approval by the majority of Parliament, Parliament which has been elected by the people and is therefore an accurate representation of the people's will through their representatives, thereby inciting to discussion among the different parties in a parliament to generate consensus. In this type of setting one cannot rule absolutely, since one has to follow the rules of its own constitution, and frequently enough most constitutions had a period of "Law Sickness" (I'm not sure if this is the accurate name in English), where if you legislated a Law of a given subject, you could not legislate a Law which would conflict, replace or override the former Law until that period of sickness had surpassed. (While in the Laws by Decree you could for instance do:
"I, King Peter, by the grace of God, hereby decree to all who are made notice that from now on all citizens shall wear high heels, under the punishment of death"
Then on the next day issue another decree directly contradicting it:
"I, King Peter, by the grace of God, hereby decree to all who are made notice that from now on all citizens shall only walk barefoot, under the punishment of death".
Of course, Parliamentarism also implies that there are elections for the Parliament, but that's obvious.
