NAND is a type of Flash Memory
Developed by Toshiba a year after Intel's NOR flash, NAND flash functions like a disk rather than memory. Flash Translation Layer (FTL) software makes flash look like a disk drive to the operating system. Reads and writes are sector-sized blocks of 512 bytes; however, typically, a 2KB page of four blocks are read and written at one time. Before writing, cells are erased in blocks ranging from 16KB to 128KB. Less expensive than NOR, NAND flash can be rewritten up to a million times, and erasing and writing NAND is faster than NOR
When creating a back-up of a NAND it is archiving each inode, every superblock, from zero to the last. (In the case of our Z's a full 128MB of installed memory) You do not need to have an OS installed to perform a NAND backup. (the service menu is not an OS per-se -- as long as the service menu functions you can reflash a NAND image) ... **as a note there are also two service menus .. in case one gets corrupted. They are accessed by hitting [D]+[M]+power and [Fn]+[D]+[M]+[Power] ( D+M stands for Diagnostic Menu)
The back-up/restore function is found in Sharp based ROMS (Cacko/Sharp) under that name .. I think it is called something different in OZ. pdaXrom does not have a utility built-in. backup/restore is OS dependent in that You need to have already flashed an OS on your Z. It is creating a tarball (compressed image) of the files you have specified. ie: your /home directory, also user space, configs, and installed programs... it generally does not archive the kernel, all system files, etc ..
To restore from a restore/backup tarball you need to have a working OS flashed to your NAND memory. (generally the same version the tarball is originally from) the tarball then extracts your /home and configs so you can duplicate your settings.
A good way to think about it is that the NAND is the physcial base like your harddisc. a NAND backup archives everything where it phsycially is on the disc. the kernel, libraries, configs, etc... all remain exactly where they were when you created the image. You do not need to have an OS flashed to restore from a NAND image. it can be used to install a full system, OS, settings and all. for instance a C1000 owner running OZ could use a NAND backup from another user to install pdaXrom fully configured with extra prorgams already installed. it will overwrite EVERYTHING.
a backup/restore generally adds or alters whatever is already there. If you do not have a working image on the Z, the restore will fail. or if you try to use a backup/restore for a different OS then what you are currently running, the restore will fail.
hmmmm ... make more sense when to use which? I haven't needed to make a backup in a long time .. pdaXrom is very stable for me ... but I used backup/restore to keep my calendar data up to date, and avoid minor inconviences.