What is a MemBlockString?

In a code from MemoryBlock (online docs

), I found:

Var m As MemBlockString

// create a New memBlockString
m = New MemBlockString

// add the strings into the memBlockString

?

Example Projects > Advanced > MemoryBlock > FastStringAppend.xojo_binary_project

Thanks MT.
They can add a string in the example sayong “MemBlockString is declared elsewhere” (or something similar), so people can understand at the first look.