Maimonides's intro to his exposition of the Mishna, the Maharitz Chiyus and many other sources go out of their way to explain that the categories are as follows:
1) There are full fledged exegetical derivations which are demanded by scriptural nuances and terminology that were passed down from Moses through the generations.
2) There are full fledged exegetical derivations which are demanded by the scriptural nuances and terminology which were discovered by later generations.
3) There are rabbinical laws that were already instituted in the days of Moses and Joshua and onward by virtue of what is called "asmachtah" - which is a lose scriptural reference (as opposed to rigid derivation) but one that nonetheless serves as a hook and a justification for the rabbinical law.
4) There are full fledged fences and injunctions which were made much later in Jewish history - some of which have known scriptural "asmachtos" and some that don't.
5) There are Torah laws that are purely oral and have no scriptural reference at all not even an "asmachtah" (this is what the Maimonides calls "halacha leMoshe miSinai - laws given to Moses at Sinai).
6) There are also "minhagim" (customs) which when legitimate are always emerging from the Jewish spirit of endearment of and deeper appreciation for the importance of devout service but which do not have any real derivation or even "asmachtah" in the classic sense but which are rooted in the transmission of the oral law in its full robust sense.
The underlying question is always therefore going to be do we and can we trust the sages, the transmitters, and the luminaries?
It is my personal experience that the closer a connection one has (and historically has had) to one of these people the more likely it was that the trust was established because the integrity and the clarity were self apparent in those relationships. The further the distance between the person and a sage or luminary of that degree the more likely or easily convenient it is to pawn them off as self-serving, dishonest or unreliable.