Apparently from various people commenting in places, some folks do not know how to do this fun thing.
So here is my own little tutorial on how to create an offsim item. The fun in these is to give regions much more depth than a single [or many] region[s] can do. There is much less need for this in OpenSim using core code which has such features as Varregions.
With pricing so low and easy entry for those technically inclined many of us can have many regions.
However, some are not able to run their own simulator instance[s], or have to slow a connection or are datastream limited, or just would rather someone else handle the simulator side, offsim creations can add much to a region.
I am not quite certain [and could not find a definitive answer in a quick search... perhaps I will add it here later] the distance one can link prims. It is possible there is no limit.
I do have one out now that the offsim object is around 150m from the inregion root prim, so I kinda suspect it can be as far out as 256m from the region root prim. But that distance is rather not one to use, as, unless it is a very big build, you really cannot see it enough, if at all, to do it justice, especially for visitors.
Also note that you can push any items out from a region up to having 51% of it inside the region borders.
Keep in mind these are for effect only, for decoration...they cannot be used, nobody can go to them.
In any case, let's get to this. I don't do this as some kind of full tut on the matter, just something quick and easy for those already comfortable with even the most basic building tools...which is kinda what I am. If in comments in places I get better info I will just add it in here.
In this first photo I show that I rezzed this nice castle out that Kahn Khalim created that I found someplace recently. I show the root prim rezzed out.
Generally I rezz all this out above the rest of my region just to make it easier to see what I am doing. You might find others ways that are easier for you. I rezz them close to the part of the region I will be extending it from, and the root prim is usually only 50 or so meters distance at first.
You see in the below photo the castle is placed as I want it. In order to get more distance or better positioning simply edit the entire build and select linked edit the root prim.
Then you can move it a further distance inside the region or move it up or down as needed. Then you can push the entire build out further, you see.
The build can be positioned only as far the root prim stays inregion to anchor it. In this photo where you see the multi-colored dancefloor is near the corner edge.
As noted in the below photo once you have all in place just cam around it to confirm placement is what you want to see, and others to see.
In this inregion snap from a little higher than a regular walker would see, that while the castle is actually rather large, it shows up in the distance, nestled among hills with lots of water around it.
I like it and it also fills an area where I had just mountains and rocks to fill it out more.