Dear Houdini friends - can I please check if the following notes are correct. As I'm learning H I'm writing my own notes, as I guess many of us might do, and the whole area of world space / object space relationships is an important one for me. I'd therefore just like to get confirmation that my understanding - see my notes below - is correct, or not! If not, I look forward to some corrections. This is such a fundamental matter I really want to make sure I have it right. Thank you.

(I've looked, but failed to find a similar explanation in the docs - if there is one, please let me know!)

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A geometry object node (a geo container) has a centroid, a pivot - and a handle.

The Centroid defines the centre of the Geo container.
The Pivot defines the centre of transformation for the container.
The Handle defines a temporary centre of transformation for the container.

By default the Pivot and Handle are co-located at the container centroid, but both can be moved, independently.

An object’s pivot might be moved to match the centre of action for a mechanical linkage.
An objects handle might be moved to provide a temporary centre of action for object transformation.

Note that in the Geometry context the centre of object space will be based around the container Centroid - not the pivot, nor handle.

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101 views and no response... so that leaves me pondering:

1. Is this a stupid question?
2. Does no one really know? I find that very hard to believe.
3. Other suggestions on a postcard please...

What I really want to confirm: what defines the location of the geo container?

your explanation seems technically correct, may make sense to some, may be confusing to others
since there is so many intricacies that it's difficult to describe everything as there may be more things coming to play that one realizes, even though most of the times they can be ignored for simplicity of explanation

so from my unerstanding:
each object has world space transform which defines the transform of the Object (in your words "geo container") in world space
this transform consists of Parm-Transform (TRS and Pivot parms on the object), Pre-Transform and Parent-Transform (if for simplicity you don't count some other transform properties or Constraint transform overrides)

"centroid" is a location of object's origin (object space {0,0,0}) in the world space

"pivot" values represent object space offset from it's origin and serves as pivot for parm transform scale and rotation before even Pre-Transform or Parent-Transform are applied
Technically it probably just a traditional pivot contributing to the final Parm-Transform matrix

"handle" transform is an independent world space transform that serves for manipulation of Object's world transform (which contributes to Parm-Transform considering its Pre-Transform, Parent-Transform and Pivot and is decomposed to TRS) whenever Attached to Geometry

snapping handle to centroid will snap it to described centroid world space location
snapping handle to pivot will snap it to pivot's world space location

inside of the Object you are in it's Object Space so all coordinates are in object space completely independent of Pre-Transform, Parent-Transform, Constraint Overrides, ...

in general it's best to avoid introducing any world space transform to Objects where you are performing simulation unless its an intention to have object space sim
also it's best to avoid non-uniform scaling on Objects

Many thanks Tomas - I really appreciate your response. It sounds like my notes are broadly correct at a basic level, but I shall certainly study your response : )

Hopefully this little thread might be useful to future newbies, too.