In fact, you can use any line of the FS section to assign a different whatever to a certain variant.
Presumably the Manufacturer and Type won't change, and each combination of model + panel + paint + whatever will be identified by a Variation name.
Important: while what you see on your menu is what's listed under UI_Manufacturer, UI_Type, UI_Variation, FS doesn't look at those lines. FS looks at is the Title= line. So even though you don't see the Title on screen, every variation must have a different Title as well as a unique Variation name. If two variations have identical Titles, FS won't see the second one and won't show it to you on your selection menu.
You can use the UI section to select a model, panel, sounds, AIR file, checklist, etc., etc., etc. The only thing an aircraft folder can't have more than one of is the Aircraft.cfg file.
Also, you don't have to have a copy of, for example, a sound folder or panel folder inside the aircraft folder of every plane that uses those sounds or panels. You can alias the panels or sounds from different planes with entries in the Panel.cfg or Sound.cfg files. An example looks like this:
[fltsim]
alias=Douglas_DC3\sound
The above is the complete contents of a Sound.cfg file. It tells FS to use the contents of the sound folder in the stock DC-3 aircraft folder with this plane.
You can file tutorials on aliasing on any of the download sites. It's pretty simple. There's a folder in your main FS9 folder called "fsfsconv" (or you can make one there if it's not there already.) It's a pseudo "aircraft folder" where you can place panels and sound sets that you use with more than one plane, then you know right where they are when you write the alias line in a cfg file:
[fltsim]
alias=fsfsconv\sound.propliner
That tells FS to use the contents of the sound.propliner folder inside the fsfsconv folder.
BTW, fsfsconv stands for Flight Shop Flight Simulator Converter and it goes back at least to FS98, probably further. You could give your folder any name you want; I stick with fsfsconv because I'm something of a traditionalist.