Thanks for your thoughts on this. I have done some further investigation.

I now summon the marker entity by the command block in spawn. It seems if I then tp or teleport that entity to a specific place whether using relative coordinates or not it will stay available. If I tp it to the player that has died it doesn't stay available even though I summon it in spawn. Tp or teleport doesn't seem to make any difference to persistence if you use coordinates. This is true whether entered by command block or directly.

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This is what I think you were suggesting and if I do it

/execute @a[score_HasDied_min=1] ~ ~ ~ /teleport @e[name=Back,tag=AddID] ~ ~ ~

it doesn't stay available, although it does take you to the right place. I have tried adding in an offset but that makes no difference.

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if I do
execute @a[score_HasDied_min=1] ~ ~ ~ tp @e[name=Back,tag=AddID] @a[score_HasDied_min=1]

brings the marker out successfully but it doesn't stay available so I may as well not bother with the execute at the beginning.
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and if I do

execute @a[score_HasDied_min=1] ~ ~ ~ tp @e[name=Back,tag=AddID] x y z

it would stay but I don't know x y z.
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Any further thoughts. My next idea is to just keep teleporting the marker to itself and see if that keeps it available, though I have run out of time to 'work' on it today.

Some blockheads!

@e(x=0,y=0,z=0,dx=m,dy=n,dz=p) will tell you if an entity is in a box m by n by p with the north west corner at (0,0,0) I called this test the volume values test which was not a great name for it, sorry.

If you set (x, y, z) at the most negative point on your world, and dx, dy and dz to the size of the world in the respective directions then you can use this to check if the entity is inside the world border.

If anyone else is reading please feel free to add your comments and queries about commands and command blocks.

Not sure about the execute and entity selection. I may have made a mistake there, needs further investigation I think.

The vanishing may be something to do with the persistent setting. Just an idea.

Getting coordinates on the scoreboard is definitely useful as you can effectively reverse the process. Haven't implemented it yet but pretty confident it will work.

Yes the video i saw showed all that with the hopper and the minecart. Tping an entity to a chunk does seem to have some interesting properties - you can tp the selected entitiy out again and you can tp to it albeit briefly but you cant run /execute at the entity. It is not a lazy chunk - as the mc edit shows it doesnt exist as such. A pre creation state maybe, which is 'unloaded' when other chunks are unloaded. Or is it just that if you leave the entity there long enough it will create the chunk and thus it is just being unloaded.

Im sure you are right that if you use the player it will create the chunk in the file even when just passing through.

Hi there @NutjobBob

I can find the coordinates ok now but you raise some very interesting stuff and I will probably adjust my method. What I had found was that by setting the volume values test appropriately you can check whether the border has been passed. This would have to be adjusted if you change the world border.

Slightly puzzled as I actually found you can tp entities outside the world border or at least a world border you have set yourself.

It may be that you cant set it beyond the default of 60million he refers to. He also refers to an operation you cant do only when you are outside the border to test whether you are outside the border. I cant find this - only ones that always work or always don't.

If you are referring to the default border and it wont let you past, then actually my thinking is this will make it much easier as you can just try reducing steps until you can't move anymore recording as you go along. No need for backwards and forwards. The fact that you found from MCEdit that it doesn't create chunks getting there means I can use it without worrying about file size. (Phew)

As far as loading the chunks is concerned I did find a video that shows there are various levels of loading. I think there is a bare minimum load state as I found I could tp to the entity for a similar time window as for the back command referred to earlier.

Thanks @pand1024 that is interesting.

It wont help me for just keeping a chunk open for a short time as i dont want to have to put a load of stuff down that i then have to remove. It will be useful if i want to add command room further out from spawn. Of course the alternative is to save the back coords which is essentially a saveable warp.

@NutjobBob essentially this works out how far the border is by moving an entity recording and improving the approximation as you go. I am not sure whether that will force chunks on the way to be created which would be an issue for me with limited map storage. I dont think you need to use chests to store the save though - I think you can use the scoreboard and deduct amounts in the same way to go back. (If you tp the player will they see what is on the way or die in lava etc?) Quite a challenge to say the least. I also have a memory @CraftyMyner said there were over 200 blocks in the warp system though that includes vote credit charging and tests for team to see if a charge applies.

You can name them and tag them in the same way but if the same command triggers it will name them all the same whether they are clouds or armour stands. And you dont want to test on each players name separately. You have to look to the scoreboard.

Thanks very much.