The Resolve "Render Cache" is described starting page 205, in "Chapter 6: Improving Performance, Proxies, and the Render Cache". It's the same one used by Color, so you're probably already familiar with it.
As Bryan says, it's quite separate from Fusion's own "Disk Cache". Personally I haven't used the Fusion Disk Cache at all yet, because when I tried it it seemed to require rendering out the entire composition for the given nodes, which didn't suit my workflow.
I've found the Render Cache somewhat useful for Fusion-in-Resolve work.
In brief:
A. You choose the format in Project Settings -> Master Settings.
B. Options include Uncompressed, ProRes and DNxHR.
C. There's no option for reducing resolution, I believe it always caches at full resolution. But you can choose a lower quality to save disk space at least (not sure if it saves much render time)
D. There are three Render Cache options: None, Smart and User. Their various permutations are a bit involved, so read page 209 for full details, but generally choose User if you want more control over what is cached. This is also affected by some tickboxes in the same area of Master Settings, which decide when background caching occurs (default: after 5 seconds of idle), whether all Fusion effects are automatically cached, and also have options for caching Edit page compositing and transitions.
On top of that there's Proxy -> Off/Half/Quarter. This only seems to affect live playback, both on Edit and Fusion pages. I've found this useful for live playback on the Fusion page, where dropping to Quarter will increase the FPS of my current comp from 4 to around 10. I guess this is roughly analogous to Fusion Studio's Proxy setting, except with only the "2" and "4" options.
In the project I'm working on, I have resource-intensive 3D Fusion effects for 100% of my runtime. If I want to do live playback I can get 10 FPS if I drop to Quarter, but that's still not fast enough to get a decent feel for animations and the like. Plus I can't hear sound on the Fusion page (not that it would sound great at 10 FPS anyway).
As a result, I've been using the following caching strategy:
1. Render Cache format set to ProRes 422 HQ
2. Render Cache mode set to User
3. "Automatically cache Fusion Effects in User mode" : No
4. "Enable background caching after" : 2 seconds
5. On the Edit page, I have "Bypass Fusion & Grades" on most of the time, so I see no Fusion output in the viewer. This enables me to scrub very fast through footage. I have a shortcut assigned to this toggle so I can quickly switch between on and off when I do want to see the Fusion output of a given frame or section from Edit.
6. When I need to see the Fusion output for a section, eg to check my camera moves are OK and lined up with audio, I use the Edit page razor to cut out a small clip that contains the area I want to see.
7. Then on that clip I choose Render Cache Fusion Output -> On. This gives me the red bar at the top of the timeline, and I sit and wait for it to turn blue. Once it's blue I can disable "Bypass Fusion & Grades", and can now watch the cached section at 30 FPS with full Fusion output.
8. Once I'm happy that the section is OK, I usually then remove the cuts (Delete Through Edit) so I don't have more clips than I need, and set Render Cache Fusion Output -> Off again so I don't end up caching material I don't need for now. Then I usually re-enable Bypass Fusion to get back to fast scrubbing. I sometimes toggle this off on the Edit page to check individual Fusion frames.
An example of how I cache: I want to see the Fusion effects across the boundary of these two clips, so I have cut out a small section from each, and set Render Cache Fusion Output to On for both. The red bar appeared above them, and is now turning blue as it caches.
This is working reasonably OK for me, though it's definitely not ideal and I wish there was a way on Edit to choose an arbitrary range for caching, so I didn't have to keep cutting sections out of clips to cache them.
One thing I should add is that at least on my system, I have been unable to get reliable caching within Resolve's Fusion page. When I hit play I see the green bar on the Fusion timeline, but it seems unable to cache more than 2-3 seconds at once. After it reaches a certain length, the green bar starts moving along the timeline rather than continually expanding.
This is probably because my 48GB RAM is not enough for Resolve + Fusion with the comp I have, especially as Resolve's settings won't let me allocate more than 36GB in total to Resolve, which limits Fusion to 27GB.
This limitation has made me much more reliant on Resolve's Render Cache, which caches to disk and therefore can cache any duration. Plus I usually want to hear the audio at the same time anyway.
A final point to note: I have found some unreliability with the Render Cache in terms of when it will start caching. As mentioned I am in User mode and am setting individual clips to Render Cache Fusion Output = on. The expected result is that as soon as I do that, I see the red bar at the top, and then within 2 seconds that bar starts turning blue (because my Background Caching setting = 2 seconds).
However, sometimes the red bar appears but never starts turning blue, no matter how long I sit idle waiting. Other times, less frequently, the red bar doesn't appear at all.
In both these situations I have to:
1. Turn the current clip's Render Cache Fusion Output to Off
2. Set Render Cache to None
3. Set Render Cache back to User
4. Set the current clip's Render Cache Fusion Output to On again
This usually fixes it. If I try doing only steps 2 & 3 it sometimes fixes it, but not always, hence I now always do all four steps. Annoying!