After a couple of searches .
The difference between UEFI Boot and Legacy boot is the process that the firmware uses to find the boot target.
Legacy Boot is the boot process used by BIOS firmware. The firmware maintains a list of installed storage devices that may be bootable (Floppy Disk Drives, Hard Disk Drives, Optical Disk Drives, Tape Drives, etc...) and enumerates them in a configurable order of priority. One the POST procedure has completed, the firmware loads the first sector of each of the storage targets into memory and scans it for a valid Master Boot Record (MBR). If a valid MBR is found, the firmware passes execution to the boot loader code found in the MBR which allows the user to select a partition to boot from. If one is not found, it proceeds to the next device in the boot order. If no MBR is found at all, the user is presented with the famous "Please insert system disk yadda yadda yadda"
UEFI boot is the boot process used by UEFI firmware. The firmware maintains a list of valid boot volumes called EFI Service Partitions. During the POST procedure the UEFI firmware scans all of the bootable storage devices that are connected to the system for a valid GUID Partition Table (GPT). Unlike a MBR, a GPT does not contain a boot loader. The firmware itself scans the GPTs to find an EFI Service Partition to boot from. If no EFI bootable partition is found, the firmware can fall back on the Legacy Boot method. If both UEFI boot and Legacy boot fail, the famous "Please insert system disk yadda yadda yadda" will be presented.
In general, UEFI boot is more desirable.
So i let the options as the Default choice (Legacy in my case).
The steps of my installation :
- Shrink one of my partitions in windows 10 disk management (30GB) and creating a new partition (E: like exemple).
- Create a bootable USB Using rufus (elementary OS Loki 0.4)
- Restarting the computer and boot from the USB
- install elementary OS ... Choosing the language connect to network (optional)
- In the choices of installation in my case i didn't find install alongside win10 so i chose something else
- I deleted the partition that i create in step 1 and now i have a 30GB free space, i created one partition for swap (2GB in my case) and one for /home (5GB in my case), the rest for the / partition .
and i can dual boot elementary OS Loki Alongside win10