I have a multiboot boot Windows system with Windows 7, Windows 10 and Linux partitions.
I installed Macrium 7 on my Windows 7 OS as I use that most often. However I am about to create rescue media and I am wondering if the PE 10 rescue is the best for my situation.
The question is: Will a Windows PE 10 environment work for older images created with Macrium 6, and can it boot in either MBR or UEFI BIOS systems?

About WinPE 10 and Reflect 6 images, yes that will work, although that has to do with newer versions of REFLECT still supporting images from older versions, nothing to do with WinPE. And yes about UEFI+BIOS support, which is supported from WinPE 4 onward.

Typically you want to use the latest version of WinPE available to maximize compatibility and hardware support. The only reasons you might want to stick to an older version are:

- if your Rescue Media will require additional drivers, in which case matching WinPE and "main" Windows versions allows Reflect to automatically add drivers from your main system to your Rescue Media build. However, sometimes newer WinPE versions have appropriate drivers already built-in when older versions won't, and even if not, often the same driver works from Win7 up through Win10 anyway. And as a last resort you can always manually download and provide Win10 drivers for your hardware during the wizard just for incorporation into Rescue Media even if you're running Win7.

- or if you have a system that doesn't work properly with WinPE 10. Some Lenovo and other systems that have non-compliant implementations of UEFI end up locked at 640x480 resolution in WinPE 10 when booted in UEFI mode, and that makes Reflect's interface unusable, but often those systems work fine with older WinPE versions.

Ok, thanks for that reply. I noticed when I built the rescue there is a grayed out checkbox that says something like,
Enable multiboot MBR/UEFI BIOS booting
Why is that grayed out regardless of PE version?
I built rescue ISOs for PE 3.1, PE 5 and PE 10, it it was always grayed out. The only one I didn't build is for PE 4

It might depend on the target you select. Discs and ISO files include support for both types automatically, so if you select those it will be grayed out and checked, forcing it on. If it's grayed out and UNchecked AND you're choosing a USB device as the target, I'm not sure.