A newer version of the documentation might answer your questions .
Octave's default index type is a 64-bit integer for a 64-bit machine.
You can easily create big matrices in your memory. But Octave's default
Fortran index type for library calls (BLAS, LAPACK, SuiteSparse, ...)
might remain a 32-bit integer, due to the fact how those Fortran
libraries are distributed these days. This limits the actions you can
perform on those huge matrices you can create in Octave.