Other than the Proteus Provectus, which has solid conductors, the rest of the Purist line (as you go up the model range) varies as follows:
1.) Conductors: total wire gauge (AWG = American Wire Gauge)) as caluclated for EACH conductor (signal, signal return, and ground) increases. This applies to IC's, speaker cables, and power cords.
2.) Conductor material: As you go up the model range, the metallurgy of the conductors becomes more sophisticated, eventually incorporating alloys of gold, silver, and copper in various combinations.
3.) Conductor topology: refers to the number of conductors (and insulators/separators) and the complexity of their arrangement within the jacket. Both increase as you go up the model range.
4.) Regarding the Fluid, Ferox, and Contego attributes:
a. -- Fluid (only,) makes the most managable cable (of the three) especially speaker cables and power cords. It provides good mechanical damping (conductors vibrate when they carry a signal) and (less important) isolation from airborne or floor vibration.
b. -- Ferox is a kind of 'paste' made of iron oxide, In addition to providing the mechanical attributes of fluid jacketing, it offers additional EMI/RFI shielding. The downsides are: heavy and not as 'bendable' as fluid alone, and the proximity of such a mass of iron-bearing material would have to affect the inductance properties of the cable. IIRC, some people who have extensive experience with both, say the Ferox cables are a wee bit darker sonically. Not an issue with my system ;--)
c. -- Contego: is a mixture of the two as I understand it, and presumably provides some of the features of both, i.e. manageabiliy AND shielding.
One quality all Purist owners mention, regardless of model, it the utterly SILENT background of PAD cables (most important with IC's of course.) I feel this is due to the way the jacket material absorbs (internal) mechanical vibration (from the conductors.) Further, the fact that the jacket diameter gets larger and larger as you go up the model line, is more than affectation, since larger conductors need more mechanical damping.
I still think the Venustas cables represent the most VALUE in the Purist line; in as much as it incorporates MODERATELY sophisticated conductors (both the metals, and the topology/layout) combined with enough jacket material to provide good mechanical damping and that super-quiet background.
Hope that helps in understanding the how/why of the inreasing complexity of the various cables' construction as you go up the model line.