Boot files (bootmgr and BOOT folder with BCD) can be placed on SYSTEMDRIVE. A separate partition was created for the purpose of increasing the security and reliability of the OS booting, and it is necessary for some cases, eg for the run Bitlocker.
Partition where the boot files are stored should be the primary and active. The partition number is not important.
So if I understand your reply, as long as you're not using Bit locker,
booting from the partition that has the OS files (System Drive) is permissible like booting from a separate System Reserved partition as long as the booting partition has the boot files and is marked Active.
Also, there is no specific order to the partitions and you could have the booting partition be either first or second or third on the hard disk.
Do I understand this correctly?