Thank you for your time and your answer.

For gpt disk with bios boot:

You need a 1MB or greater partition with no filesystem ("unformatted" in gparted, way at the bottom of the list)
and that partition needs a bios_grub flag (EF02 in gdisk). I've been putting this partition at the end.
You'll need at least one linux-formatted partition for the operating system.
You do not need an efi partition and you don't want the esp and boot flags on the linux partition. If you happen to have a real efi partition in addition to the others, it will just sit there and do nothing until you boot in uefi mode.

If you want to dual boot gpt disk with bios, you'll need another linux partition for the second linux installation. If you don't let the second one install a bootloader, you'll need to run update-grub on the first installation to add it to the boot menu. Either linux can be in charge of booting as long as you have a menu entry for the other one. (unless you're a fan of grub command line.)

For uefi boot:

You need a fat32 partition, around 50 - 500MB, with esp and boot flags. (Hint, just check esp in gparted and boot will get checked automatically.) (Hint2: I have 3 installations on one hard drive and the 200MB efi partition has 196MB free.)
You will need at least one partition for the OS, with a linux format and no flag.

If you want to dual boot on a uefi setup, you'll need another linux partition (exactly same advice as above regarding boot menus.)
The difference here is that the you will accumulate bootloaders in the efi partition. A different one for each distribution you install. Run efibootmgr to see the boot order or to make changes. Read the man page and especially read about uefi bootloaders at rodsbooks.com
http://www.rodsbooks.com/efi-bootloaders/index.html

Edit: Ohhh... too many words up above. Read it anyway.

I need to understand more about Devuan jessie installation on different partitions.

Which is the right way to install Devuan on msdos and gpt (non efi / efi) partition table? I want to fix single boot and dual boot.

Let's suppose that i have 30 Gb of free space, 1 Gb ram memory on disks for each case.
(for dual boot I believe I can resize the partition first of the primary o.s. with gparted, that's the tool I am using but any other suggest for cleaner installation will be appreciated)

#1 Case: Installation for single boot 32bit or 64bit on PC doesn't need uefi, so:

Partition table: msdos

/dev/sda (mbr )
/dev/sda1 (29 Gb, formatted ext4, boot flag)
/dev/sda2 (1 Gb linux-swap)

Partition table: gpt

/dev/sda1 (legacy_grub, 150Mb)
/dev/sda2 (29 Gb, formatted ext4, esp boot flags)
/dev/sda3 (1 Gb linux-swap)

#2 Case: Installation for dual boot 32bit or 64bit on PC doesn't need uefi

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#3 Case: Installation for single boot 64bit uefi mode

?

#4 Case: Installation for dual boot 64bit uefi mode

Thank you for your time guys. You are cool!!!