This has always been a problem. In the early days, the manual contained some guidance on these points, but there is not currently any guide to tell users what each field is designed for. I’ve still to work out what “Packaging Method” means.
The staff at EndNote, who write the output styles, are not clear about this either, so there are all sorts of odd inconsistencies in the styles.
In practice, you have to select your output style, then open it for editing, and see how it is configured to handle the different fields. If necessary, you can edit the style, and even modify the reference types, to get the output that you want. But this requires a fair bit of investment of your time in learning the software. Most users aren’t prepared to do that.
As for your specific queries:
Series Volume: Use for the number in a numbered series, e.g. Studies in Illinois Archaeology 6 [the numeral 6 would be put in the Series Volume field]
Volume: Use this field when you are creating a reference for an individual volume in a multi-volume set. This is particularly applicable where each volume has its own title, e.g. Pelikan, Jaroslav. The Christian Tradition: A History of the Development of Doctrine. Vol. 1, The Emergence of the Catholic Tradition. Chicago: University of Chicago Press, 1971.
Number of Volumes: Use this field when you are creating a single reference for a whole multi-volume set, e.g. Byrne, Muriel St. Clare, ed. The Lisle Letters. 6 vols. Chicago: University of Chicago Press, 1981.
Conference Paper: I only use this reference type for unpublished conference papers. Published conference papers should be entered using the Journal Article or Book Section reference type, as appropriate.
Conference Proceedings: I try to avoid this reference type. Normally a published volume of conference proceedings can be entered as an Edited Book. There are some bibliographic styles that require extra detail when citing a volume of conference proceedings, so I would use this reference type in those cases.
University of Queensland Library