The RenderTarget class is just a class that creates Frame Buffer Objects (FBO). You can render a plane to a render a target instead of your canvas and then use that render target either by assigning it to a ShaderPass or by grabbing its content (ie its Texture) and use it in another plane.
Please note that render targets (and therefore shader passes) disable the WebGL context default antialiasing. If you use them, you should set the antialias Curtains property to false when initiating your context.
You might then want to add an extra antialiasing pass, like FXAA or MSAA. See the Post processing scrolling wheel with custom transform origin for an example of how to add a FXAA pass.
Using a RenderTarget texture in a plane
Assigning a RenderTarget to a ShaderPass
You can decide to add a ShaderPass to a defined set of plane instead of your whole scene. By combining a shader pass with a render target, you'll be able to add an additional pass to all the planes that use that render target.
Check the Selective shader passes using render targets example to see how you can apply different shader passes to different bunch of planes.
index(integer): v5.0 read only
The index of the render target in the renderTargets array.
An array of length 1 containing the only render target's Texture.
userData (object): v5.0
An empty object to store any additional data or custom properties into your render target.
uuid (string): v6.0read only
The render target's unique identifier.