World coordinates: References the build plate with 0,0,0 at your defined "home" position on the build plate. For example, on most Prusa printers this is front-left of the build plate. If you have an Object selected then the displayed position coordinates be the center of the bounding box of the first Part of the selected Object. If you have a Part selected then the displayed position coordinates will be the center of the bounding box of the selected Part.

Object coordinates: References the the Object with 0,0,0 depending on what is currently selected. If the full Object is selected then 0,0,0 is at the center of the bounding box of the original location of the first Part. If a Part is selected then 0,0,0 is at the center of the bounding box of the first Part of the Object. If you have an Object selected then the displayed position coordinates be the center of the bounding box of the first Part of the selected Object (0,0,0). If you have any Part selected then the displayed position coordinates will be the center of the bounding box of the selected Part relative to the center of the bounding box of the first Part. (Hmmm... my playing around in PrusaSlicer to make sure that I'm being accurate exposed a complication. If you move only the first Part of an Object, the Object's center becomes decoupled from the first Part's center. We'll need to play around a bit more to fully grock what is going on. And this is even before changing the order of the Parts within the Object...)

Part coordinates: Only available if a Part is selected. References the Part with 0,0,0 at the center of the bounding box of the selected Part. Will switch back to World coordinates if you click off of the Part. The displayed position is the center of the bounding box of the selected Part, so will always show 0,0,0. This is useful for moving a specific Part a specific distance by typing in a number in x, y, or z. Once you press Enter, the Part will move that amount and the displayed position will again reset to 0,0,0. Useful to move a Part without having to do the internal addition or subtraction yourself.

Even with the complication exposed in the Object coordinate space, if you know the dimensions of your different Parts this can help you get relative positioning of the Parts within an Object without having to continually add and subtract the world coordinates.

Deadpool Remixed for MMU Example:
1. Download the five color-labeled STL files: https://www.thingiverse.com/thing:4827379
2. Import (CTRL+I) only the black color file in PrusaSlicer: deadpool-Black-V3-icz.stl
3. In right-side PrusaSlicer pane, there are three coluns: Name, Extruder, Editing.
4. Click imported object: Click under Editing column, Click Add Part, Click Load...
5. Open another color-labeled STL file: deadpool-Brown-V3-icz.stl
6. Repeat steps 4 & 5 until all color-labeled parts are loaded, for Red, White, and Silver
7. You're done. All colors added as parts, and all parts have been auto-aligned.

TY

Just installed an XL profile and tried it, first loaded part seems to jump to some other location (either middle back if there's a wipe tower or front left if none) but parts 2+ all seem to go to the centre as they do for other printers.  So prusa is doing something different for the XL.

also

>>However when you load an object it automatically gets placed in the centre of the bed.  So if you load multiple objects in turn then they are all placed in the middle and so centred. 

Is not true.  Don't know the innards of the .STL file, but I use the free Tinkercad (hey, I'm retired - it's easy :) ) and it does not automatically load it in the center of the bed, as I just verified.  But per above: World Coordinates of 180/180 for the XL will center it in a second!

However when you load an object it automatically gets placed in the centre of the bed.  So if you load multiple objects in turn then they are all placed in the middle and so centred. 

How? I couldn't find any  'Center' option, unless you mean doing a Manual Calculation of X/Y location on the lower right Pane.

TY

Henry

I found your explanation of "Objects" vs "Parts"  EXTREMELY helpful.

And I took the time to edit it a bit, and add explanations that a 'dummy' like me finds helpful.

I don't frequent the forums much (I'm a 67 yo retiree whose Mech Eng son got me into this!), but if you ever have reason to help others as you have helped me, may I offer this slightly modified explanation of yours.

TY

Difference between Objects and Parts?

Ok here’s a basic run down.

Each OBJECT is the root of a branch in the right hand window pane.   

Within each OBJECT can be multiple Parts.

OBJECTs are the Assembly, and Parts are the components

Parts appear as branches of its parent OBJECT in the right hand pane

(This is like folders in Explorer; but unlike Explorer there are only these 2 levels in the Slicer). 

Each OBJECT is sliced and processed individually. 

+Do not attempt to overlap  OBJECTs, or bad things happen.  

+ OBJECTs must be placed on the print bed. 

   So trying to lift an OBJECT above the print bed is not allowed, it will just drop to the bed. 

   But as long as some piece of the OBJECT is on the bed (or raft if enabled) then its ok.

You can select Parts only by clicking on them in the tree view, not the Plater view. 

   If you try and grab it in the Plater view, it will just grab the whole parent OBJECT. 

Unlike OBJECTs,  Parts can be moved around and can be floated above the bed. 

(think of it as if the ‘Part’ is resting on the ‘OBJECT, and the OBJECT is resting on the print bed)

Also overlapping Parts are sliced properly and clip each other.

SPLITTING:

Splitting into OBJECT basically is destructive: It turns all pieces into root-level OBJECTs,

(like if you had the number ‘8’ as a cutout, it would create each circle in the ‘8’ as a new OBJECT.)

And as an OBJECT, all the individual pieces will all fall to the bed, since they are now a bunch of disassociated models. 

Unless there is some overriding reason to split to objects then it’s best not to.