yep, the "minimum number of hops from supplying system", so you can get a quick visual overview of which systems you can supply easily, and which are more out-on-a-limb, i.e. to visualise the supply radius.
I don't think this would be very useful... As long as you can see the supply-carrying lanes, it doesn't really matter how far you they are from the nearest source. (If there's a bottleneck, it will be visible either way...) There are also a few other hard-to-see and somewhat complicated situations that could arise, but this suggestion wouldn't help clarify them.
Also, it's not very scalable as supply radius grows. If you have some very well developed supply sources that can supply 8 or so jumps away, then the colouration of the lanes will be rather dim at the extremes, and the distinctions between different numbers will be hard to see easily. Easily-visible drops of 30% per jump only works up to 3 or 4 jumps.
More useful to show might be the max amount of supply that can be provided to ships in a system, if we end up limiting this.
The "two systems" bit referred to the contrast between the first image in the thread ("eleazar's system" ?), which clearly only shows supply lines in use between actual colonies and fleets, and the work in progress image you showed which shows all systems which could be potentially supplied out to four jumps from the supply source.
It's probably just becase it's out of its original context
, but it's not clear from eleazar's image and accompanying discussion how one decides which starlanes are "in use" as supply lines. In particular, in some cases, systems two jumps apart have coloured lanes through uncolonized systems between them, but in other cases, there is no colouration along a seemingly equivalent path. There's mention of not manually creating supply lines for ships, which implies that the rest was assuming explicit manually-created supply lines
between systems, which might be the explanation.
(Discussion of details of how to decide what supply routes should exist or how to pick them or whether the player should pick them are best kept to the above-linked threads.)
Regardless, the practicality of using a system that looks like eleazar's mockup likely depends on the assumptions that went into it, so it may not be practical to use such a system in the actual game.
If systems that can be fleet-supplied and systems in which the player has some visibility are the same though, we'll probably want to have some indication of which systems these are and which starlane connections cause the systems to be supplyable / visible. If there are also a separate classification of systems or starlanes that do or could connect systems for planet / resource distribution purposes, we'll probably want to indicate that separately.