It doesn't scan a whole runtime, ever, except at startup or when you add a new runtime.

Refresh happens on a folder.

Say you have 3 runtimes added to Poser: Poser 8, V4, and M4

In each of those runtimes there are 9 library folders:

Poser 8/Runtime/Libraries/Character
Poser 8/Runtime/Libraries/Props
...
Poser 8/Runtime/Libraries/Materials
etc.

V4/Runtime/Libraries/Character
V4/Runtime/Libarries/Props
...
etc.

M4/Runtime/Libraries/Character
M4/Runtime/Libraries/Props
etc.

Now when you look at the GUI, and you are viewing the Figures category, you see 3 top-level items in that tree.

Poser 8
V4
M4

Each of those is just one of the 9 folders in a runtime, but it is listing all 3 runtimes. Those do not repesent the entire library, just that one category folder.

Poser 8 = Poser 8/Runtime/Libraries/Character
V4 = V4/Runtime/Libraries/Character
M4 = M4/Runtime/Libraries/Character

Similarly, if you're viewing the Props category, you see 3 top-laval folders, which are the Props folder from each of your runtimes.

Poser 8 = Poser 8/Runtime/Libraries/Props
V4 = V4/Runtime/Libraries/Props
M4 = M4/Runtime/Libraries/Props

If you hover the mouse over any of those, it will show you the exact folder it represents.

When you refresh one of those top-level items, you are not refreshing the entire runtime. You are only refreshing one category in one runtime.  Specifically, you are refreshing that particular folder, and anything under it.

If you have 10 runtimes, there are 90 top-level folders presented to you, grouped by category. No single tree represents any entire runtime, i.e. no tree has all 90 top-level folders in it.

Each category tree shows you 10 of the 90. You select one of those 10 and refresh it. No other categories are refreshed when you do that. To refresh an entire runtime, in all nine categories, you'd have to select all nine of the top-level folders in that runtime, and refresh them one-by-one.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)