Definition of a flat string and example

Thr 2018.10.11

re: http://forum.espruino.com/conversations/316941/
'You can even access data stored in flat strings:'

What is a 'flat string' and how is it different? Definition, documentation please?
Is it just a contiguous block, null terminated? Is there a size limit?

Skimmed over

http://forum.espruino.com/conversations/316409/#comment14077573

It appears that 'flat string' may be specific, as a quick search only uncovered one reference, other than Espruino.com

https://dxr.mozilla.org/mozilla-central/source/js/src/jsapi.h#4035
'Flat strings and interned strings are always null-terminated,'

Having issues with var addrsrc = E.getAddressOf(src,true);
if(!addrsrc) throw new Error("Not a Flat String");

irratically returning 'Not a flat string', loading same data each run.

Working with

var buffer = new ArrayBuffer(8);
var n = new Uint8ClampedArray(buffer, 0, 4);

and E.compiledC

Is a flat string somehow related to Uint8ClampedArray
Not mentioned however in

https://www.espruino.com/Reference#t_Uint8ClampedArray

Is flatAddress just a T/F flag and not the address of the variable? This implies that param should always be true, e.g. why would you supply 'false' if you are attempting to get the address of (E.getAddressOf) the variable supplied? Seems redundant.

https://www.espruino.com/Reference#l_E_getAddressOf
flatAddress - If a flat String or flat ArrayBuffer is supplied, return the address of the data inside it - otherwise 0

I have a hunch that the size of the referenced address passed or returned is causing pointer (my code) offset errors.