Advanced Landscape Layers
All parameters and settings should have a description when hovering over them on the editor mode. This page will cover more of the advanced parameters and explain how they work.

Texture Variation

This is the same function that was added in UE4.26 and is a nice way to break up noticeably tiled textures. Currently it is on by default but can be changed in the global defaults.
By default it will offset each tiled texture sample by the "Texture Variation Offset", it uses a noise texture to do this. Scale and Levels control the noise texture as to how it will break up the samples, tweaking these values will give you desired results depending on the type of texture you have.
Texture Variation Off
Texture Variation On
Having a Heightmap texture input and increasing the "Texture Variation Height Influence" will help break up the seams from the noise texture.
Using "Texture Variation Use Dither" with RVT can give you visible pixelated seams, this feature is best used without RVT.
Using "Texture Variation Use Random Rotation and Scale" will result in unwanted normal map issues if used with PBR textures, as it rotates all textures and causes the normal map texture to not align correctly. It is best used on stylized textures that don't use normal maps.

Global Distance Swap

As noted on the Setting Up a Landscape Layer page, this is on by default and will fade the landscape layer to a less tiled version of the current layer at a distance.
Enabling "Use Custom Global Textures" will use the texture inputs displayed under the global dropdown, if disabled it will use the same textures as the layer inputs.
The "global distance swap" has similar parameters to the standard layer parameters except it does not take in Specular, Roughness, AO or Height input. Roughness and Specular are handled as offset constants and can be edited to suit what is desired.
Under the Distance Blend drop down, the Fade Range and Fade Hardness control the non-RVT versions of the distance blend. MIP Fade Range and Hardness control the RVT distance blend. The reason these are separated is that they take in different kinds of values for what they do, the non-RVT uses a sphere mask to fade between whereas the RVT version uses the texture MIP range from the RVT sample itself.

Overlay/Cliff Parameters

These parameters are specifically used for the Overlay and Cliff layer blend types when set in the Material tab.
The parameters for overlay are broken up into two areas, Falloff and Affect Previous.
Falloff is the falloff edge between two layer transitions and can be used to break up the gradient fade, it functions similar to the height blend layer type but has more customizability. Enabling "Falloff Use Height Map" will use the layers heightmap input to transition between landscape layers instead of using the falloff texture input.

Affect Previous Parameters

Affect Previous parameters will only work if your layer is set to "Overlay" blend and has the "Affect Previous" parameter enabled within the Material tab. The way this works is if you want a noised version of a layer to paint down on top of another layer to give you automatic landscape detail.
Example of "Affect Previous" Dirt layer randomly placed on top of the Grass layer using a noise texture
You can use the Affect Previous blend texture as a whole landscape splat map, just set the "Blend Texture UV Scale" to the resolution of the landscape.
Cliff parameters are used to generate an auto-cliff style layer, it uses an overlay blend type with additional features for cliff specific. This means the parameters under the "Overlay" drop down also work with the "Cliff" parameters.
The "Cliff Slope Angle" controls the angle at which the cliff is applied to the landscape, an angle with (0,0,1) will apply the cliff to the vertical edges of the landscape only. You can change this i.e snow only falling at a certain angle.
You can also stack multiple cliffs on top of each other.
Example of a Scree Cliff Layer and a Rocky Cliff Layer