My take on the Your Items side is that in the absence of any icon (or even +/- text) you have no collapsing categories, it's just a section-header.
Unlike pre-Stram, it does group things[1], but I can't convince myself that it's related to the RHS category-button order, and so it might be first(-of-a-kind)-chosen order. Though it might still be tied to RHS order, which certainly retains a lot of the funny ordering of pre-Stram lists. I'd have been tempted to do a Food superclass that brought up various food(-related) subclasses like Seeds and Plants (on the left, separated by three other categories) and Eggs (on the RHS, almost a whole vertical screen apart from Garden Vegetables that might or might not (probably not) be emergent as Plants-category on the LHS in the current form.
Either way, there's no apparent method/use of collapsable/expandable categories on the RHS other than the scrollable Button List sets up the scrollable/filterable Item list.
(It's really hard to appreciate the full image, as it's so much more wider than the screen resolution on the machine I'm re-reviewing it on while I type in this/prior messages, and I've only just seen a prior error in my understanding. I could open it outwith the browser and scroll round, but right now I'm vertically scrolling down with the Page scrollbsr to find the in-post horizontal scrollbar, to adjust more left/rigt then scrolling back up to the image I'm trying to check. It is time consuming and a mouse-heavy task as the browser has no keyboard controls that work the in-post horizontality, that I know about.)
I can't tell if (as per current DF) selecting an item on the right makes it solely present to adjust on the left. You have item +/- buttons on both sides (disabled minus-signs for all visible RHS items where clearly no 'subscription' is made that can be reduced) so I don't think that's so any more. The Animal-Tab example shows non-zero selections at work, but in a list that has no (apparent) need for category buttons in a list. My suggestions in the other place included asking about a Search line for Animals, and further one that could perhaps filter by "Milk" or "Shear". If not that, then 'radio button' behaviour (could still be label-button, as per Items categories, but toggle-down, toggle-up in appearance) keyed to qualities such as these might act just as well for filtering those foreseen qualities of interest.
But none of that requires MephDay involvement, as it stands. Not in the *masterwork icons* realm, anyway.
[1] On embark I'd often remove all food (say) from my load, then re-add everything I still actually wanted and any other bits, in order to keep similar items together in the list. Which only ever mattered in the prep. screens, it had absolutely no purpose other than temporarily revaining an aesthetic.)
L