What is LUABOX in the registry?

[Dirk Laurie <dirk.laurie@...>]

Since 5.3.2, there are circumstances in which the table
    debug.getregistry().LUABOX
gets created. One such is
    io.popen"command":read"a"
when the length of the output is more than a certain size.

For example, I keep all my Lua downloads and builds in /usr/local/src.
The following program:
    local s = io.popen"ls -lR /usr/local/src/lua-5.3.*":read"a"
    print("#output="..#s,"LUABOX = "..tostring(debug.getregistry().LUABOX))
prints:

#output=41986    LUABOX = table: 0x7a70f0

but if I leave off the 'R', it prints

#output=7949    LUABOX = nil