Ah, sorry I forgot to address the WIM question. First a bit of background information. If you look at your Rescue Media, you'll find a file called Boot.wim. That contains a disk image that has the full Windows PE environment, including Reflect and the other utilities that are included with Rescue Media. When your system boots from Rescue Media, the contents of that WIM file are extracted into RAM and mounted as a virtual disk that gets assigned drive letter X, which if you explore you'll find contains directories like Program Files, Windows, etc.
Rescue Media Builder always keeps a cached Rescue Media file set in order to build Rescue Media more quickly and also to enable the "recovery boot menu option" for users who want to use that. If Rescue Media Builder determines that the Rescue Media it would need to build at a given time matches what's already been built and cached, then a Rescue Media build is a simple file copy operation to the specified target. But if Rescue Media Builder determines that something about how new Rescue Media would be built at the current time would be DIFFERENT from the cached file set, then it will have to rebuild the WIM to incorporate those changes. Conditions that can trigger this need include doing any of the following since the last time the cached file set was generated: having installed a newer Reflect version, having changed certain Rescue Media build preferences (in the Advanced section), the Windows RE file set having been updated (typically due to you updating to a new Windows 10 release), having new/updated drivers that need to be incorporated into Rescue Media, and probably some other conditions I'm not thinking of at this exact moment. When Reflect detects that a WIM rebuild is necessary, the reason will typically be indicated in the Status readout near the top of the interface. If there's more than one reason, I believe only one will be shown.
If Reflect determines that a rebuild is NOT necessary because the cached file set seems to be appropriate based on the current state, then you can choose to force the WIM rebuild anyway even though Reflect would normally just copy the cached files to the target. This shouldn't normally be necessary, but sometimes PCs act like PCs and you need to override their "intelligence".
The need for the opposite scenario of SKIPPING a WIM rebuild when Reflect considers it necessary should be even more rare, but it's a feature that I actually suggested that Macrium add while I was beta testing Rescue Media Builder. The scenario I described to advocate for this option was the following: A user built Rescue Media at one point, then updated Reflect and found that the updated version contained a bug that broke their Rescue Media. They don't want to roll back their entire system to just to return to the previous Reflect version, but they don't have an installer for a previous Reflect version either (and Macrium doesn't offer them). But since they've been backing up their system on a regular basis, they DO have an earlier version of the C:\Boot\Macrium folder that contains a cached Rescue Media file set from the previous, working Reflect version. They can extract those files from their backup, but then there's the matter of actually BUILDING Rescue Media from that older file set, because normally Rescue Media Builder would immediately say, "The WIM needs to be rebuilt because the cached Rescue Media file set comes from an older version of Reflect than is installed." But in this situation the user specifically WANTS to build with that old file set as-is and NOT use the new Reflect version, so I suggested that Rescue Media Builder offer a way to build Rescue Media using the cached file set as it sits, without making any changes that it would normally consider necessary/appropriate. Thus arrived the Skip WIM Rebuild option.
I actually find the new Rescue Media Builder much more intuitive and efficient. In most cases Rescue Media can be built with a single click, which definitely wasn't possible before, and I find that the interface for configuring the advanced options is much more logically laid out and makes those features more discoverable. If you want all the documentation on the new Rescue Media Builder though, that's available here