Sometime I wish my mind would stop asking questions ! Ok lets say prior to your visa expiring you get a multiple entry re-entry permit which covers your permission to stay period. And then subsequently you do a short trip but still within your visa period which extends your permission to stay a bit longer. Would the re-entry permit cover that extra time or would you be stuck with the original date ? Hope that makes sense ?

4 hours ago, britishjohn said:

And then subsequently you do a short trip but still within your visa period which extends your permission to stay a bit longer. Would the re-entry permit cover that extra time or would you be stuck with the original date ?

If you have a multiple entry visa you don't need a re-entry permit unless you are using it to keep the last entry from if valid.

A re-entry permit adds no days to your stay. It only keeps the permit to stay date it is issued for valid when you enter the country.

5 hours ago, britishjohn said:

Sometime I wish my mind would stop asking questions ! Ok lets say prior to your visa expiring you get a multiple entry re-entry permit which covers your permission to stay period. And then subsequently you do a short trip but still within your visa period which extends your permission to stay a bit longer. Would the re-entry permit cover that extra time or would you be stuck with the original date ? Hope that makes sense ?

This only makes sense if you purchased a re-entry permit to keep your current permission to stay intact because you expected to return too late to use your visa for re-entry. You then, unexpectedly, returned to Thailand within the validity period of the visa. Should that occur, I think immigration on entry would give you the choice of using your visa or re-entry permit. The visa would usually be more sensible, but since it gives you a fresh permission to stay (i.e. does not preserve the old one on which the re-entry permit was based) your re-entry permit would then be dead.