Masking Objects
Masking objects allow you to shape your fluid volume by cutting out particles. The entire process is similar to Boolean subtraction operations, but requires a certain workflow.
The blue area should be filled with particles, the white areas will cut aways.
To achieve the look from the image below go through a few steps. In the first step, create four cubes, arrange them, and specify the one you want to fill with particles (named "Fill_Object" here).
Add a "Fill" emitter and drag the "Fill_Object" cube to the emitter's "Body" slot.
Set the emitter's "Particle Distribution" to "Fill Volume".
"Fill Solid Region" must be active.
Enable the emitter's Mask > From Collision Objects option.
Time to adjust the mask objects:
Add "Collider" and "Volume" tags to the mask objects – the "Collider" tag tells RealFlow | Cinema 4D that there is an interacting object.
Select the "Volume" tags. If the "Volume Mode" has not been recognited automatically set the option to "Solid Inside".
Reset the scene (go to the last frame, then back to 0). You will see that the volumes of the three remaining cubes have been used as a mask to clip the particles. This way it is possible to create complex shapes without having to model an appropriate container.
The result is a particle volume with notches.
The best and fastest way to proceed is to create an initial state from the fluid:
Go to Scene > Cache and specifiy a "Cache Folder (macOS | Windows)", where the intial state will stored.
Go to Fluid > Fluid and click on "Create Initial State", then tick "Use Initial State".
Under Mask > Mask disable "From Collision Objects".
Reset again.