Here is the explanation in After Effects Help:
Determines whether the disk cache preferences are used during rendering. Read Only writes no new frames to the disk cache while After Effects renders. Current Settings (default) uses the disk cache settings defined in the Media & Disk Cache preferences. (See Disk cache.)
Although I agree with you that the meaning of "Current Settings" in this context is indirect and therefore somewhat confusing, hopefully this explains that the difference between the settings is "Read Only" vs. "Read and Write, but only if the Disk Cache preferences are enabled".
What I am aiming for is a way to get all of the frames that were rendered made available to the RAM previewer when editing the comp. I would so love to see that green bar of joy spanning my work area after a render.
When After Effects renders frame, it stays in RAM (or in the disk cache) until After Effects needs that memory for something else. That means that you will get what you are asking for, but only under certain conditions. Here are the factors you need to consider:
- Rendering a RAM preview may result a different frame than how you choose to render to a file. If these frames are different, RAM preview can't use the frames that were put in memory when rendering to a file. For example, if your comp is set to Half resolution and you render at Full resolution, the RAM preview cache won't recognize those frames because they're not the same thing your comp settings require. However, at that point if you change your comp resolution to Full, it should recognize the frames still in memory (which may only be at the end of your comp) and show you the green cache marks for each frame that is in memory (or blue if they're in the disk cache). Another way of looking at this is that if your Render Settings are not on Current Settings, you won't get the result you want.
- The green (or blue) cache markers may never fill your entire timeline because you don't have enough RAM to contain your entire comp, at least at the current settings.