Welcome to YamahaSynth, and welcome back to Synthesis (a lot has changed while you were away).

First, we will attempt to answer your question... starting with an analysis of the selected program.
Then we will offer you, perhaps, a different way to approach this. You can decide what works for you.

“All 9 Bars” is a B3 sound, that replicates the 9 drawbars (with individual drawbar control assigned to the Sliders)... Part 1 has 8 Elements recreating the first 8 Drawbar footages. 16’, 5 1/3’, 8’, 4’, 2 2/3’, 2’, 1 3/5’, and 1 1/3’
Part 2 has 6 Elements including the 1’ drawbar, and 5 Elements that recreate the appropriate B3 percussion, rotary noise, key clicks etc.

The program is stored so that instead of the usual Part Volumes being the target for the Sliders, they act as Element Levels allowing the performer to adjust the sound in real-time as you would drawbars. This is done with the button just to the left of the Sliders labeled [Part _ Element/Operator]. When set (and in this case, Stored) to “Element/Operator” it acts as individual Levels of each footage, the button below toggles the Sliders from 1-4 (drawbars 16’, 5 1/3’, 8’, and 4’ ) to 5-8 (drawbars 2 2/3’, 2’, 1 3/5, and 1 1/3); you select Part 2 to access the 1’ drawbar (and the output levels of the sound FX).

Toggling the [Part_ Element/Operator] button will switch the role of the Sliders from, in this case, Elements Level (pseudo-drawbars) to overall Part Volumes (one for each Part).
So if all you want to do is not have the Sliders act as Drawbars and lock in the setting, simply toggle this button to “Part” and Store your version to the User Bank. The first 3 Sliders will Control Parts 1, 2, and 3 respectively. If you [STORE] the Performance to a User location, it will memorize this change of preference for the Sliders.

Extra Credit:
It is a fact, that the way Organ tones are recreated by synthesizers, they are very costly from a polyphony standpoint. As mentioned, each of the 8 Elements in Part 1 request polyphony, whether or not you have the Level up. A B3 setting where some of the drawbars are set to 0, still reserves polyphony for itself. Contrast that with an 8 Element Acoustic Piano where each Element only requests polyphony when it’s velocity is called. A single note of an 8 Element organ uses 8 notes of polyphony in the example, while a single note played on the 8 Element piano uses 1 note of polyphony for each key pressed! (2, if a stereo piano Waveform).

When making a dual manual sound, split on the keyboard, you should probably consider removing any ‘drawbar’ Elements that you don’t need. And since you talk about locking them in, you’ll probably want to just remove them (turn unused Elements OFF). Currently by duplicating Part 1 to Part 3... each note trigger by Part 3 is eight notes of polyphony.

Now here is where the ‘use case’ comes into play... if you are using this split Part to be a full organ sound that you are dynamic changing drawbar balance... then go for it, but if you are building a left hand bass for walking or chording and are going to set like 5 of the higher footages to 0, do yourself a favor and simply delete those unnecessary Elements (Off).

Alternate method, Find the B3 sound called “Left Manual” - a Single Part 4-Element B3 program great for emulating lower Manual left hand stuff.

Hope that helps.

Thanks for the detailed reply - really helpful!
I think I already understood about the use of Elements for each organ "stop" in that type of Part, and even about turning redundant Elements off (always a good idea to switch off all unused engines). It's good to have it explained so clearly. And, yes, my left hand bass is fine as an 8080000 so 6 elements have been turned off.
What I've now understood is the relationship of the control slider positions (and Part/Element toggle) in real time to the process of saving and recalling (Element and Part Volume of) Performances. I had mistakenly thought that they functioned just like other "controllers".
I've made a reasonable "split B3" single Performance for myself, but I've now also made a Live user page with 16 different "upper manual organ presets" ranging from 8080000 to full on 88888888 with all the trimmings, retaining a "fixed" bass split on all, so I can quickly go from soloing back to comping etc., without worrying about the sliders (and SSS is really good here). I'm sure there are many other ways to build up an organ, and at some point I'll probably get into B's Knees/Organimation, but I'm glad to have some understanding of the workings of Modx, thanks to the manuals, your great tutorials and this forum.
Hope I haven't used up all my Credit 😀 -plenty more questions to come - just managed to get FC1 working on upper split only (better than on a real Hammond!).
Peter