Rootfs is limited to 256mb of memory because it's a separate chip (or whatever) that's attached to the processor if I'm not mistaken.
Every program installs a bit of itself in rootfs, some more than others, and the rest of it it installs on other parts of the device.
Now, it runs out real quick, so you have to be aware of it, a good way to keep track is to download conky and check it out once in a while.
If you run out of space, you can't install anything else.. Firmware updates require huge amounts of rootfs space (Just for installation) and updating can be extremely problematic.
To free up space in rootfs you should remove unneeded apps and try this link http://wiki.maemo.org/Free_up_rootfs_space

Which of the scans should we use to show the memory that installed apps are using? "Scan Packages", maybe? A little primer on the basic functions of how this app works might be useful.

just try
you can't do anything wrong
maybe here's already a thread about it!? don't know.

Does that mean that you don't know which scan shows what?

that means that i haven't installed it at the moment - so i can't look it up. sorry.

EDIT: but the important scan should be the root-scan. there the folders are sorted by their size.

/home doesn't really matter because this is your biggest partition with about 27GB.

/sys (~18MB) and
/lib (~15MB) is where your firmware/system files are stored.

/var (~38MB) is where your catalogue packages are stored. if this one uses much memory you may want to disable some catalogues. (i have the 4 default catalogues + testing + devel activated. that's why it's ~38MB big.)

/usr (~268MB) should have your attention:
with ../lib (~185MB) where the localizations are stored and some needed libraries.
and ../share (~57MB) where mostly themes, icons, fonts, sounds, etc are stored and some localizations.

if anyone if these or the other folders listed in the tree-view of storageusage are using too much memory (you can orientate yourself on my (~ $size MB) values) it may be something wrong. (not optified packages for example)

"Scan packages" is the one you want. The packages using rootfs space (more than some fixed amount anyway) have their usage bars shown in red. You can ignore any python stuff though, as StorageUsage hasn't been updated to take account of the optified python setup.

Thanks for the info, I thought so. The info about red bars is very helpful, too. I see I can double-click any app name to get the detailed breakdown of filesize/space using, that is helpful too and good to know.

Good for me, I have no optimization issues!

Npw this thread is the thread people can search for about info on the "Storage usage" app %^) - good for all of us!