In the chumash, there are approximately 10 places where a law is given to the Children of Israel and is preceded by Hashem's telling Moshe to command (tzav) the people, or certain people. An example is B'midbar 5:2 "צַ֚ו אֶת־בְּנֵ֣י יִשְׂרָאֵ֔ל וִֽישַׁלְּחוּ֙"
But in many other cases, laws are conveyed with other verbs (such as "va'amarta", Vayikra 19:2). In a few cases, both a tzav-rooted AND an amar-rooted word are used (B'midbar 34:2).
Is there a difference in the nature of the law which would call for its being framed by a tzav-based word vs. any other word?