Mari 4.6v4 docs:

Multi-Channel Layer Workflow Rules

Note: These rules are only for layers and not applicable for Node Graph workflows.

Shader Models

Channels containing multi-channel layers should be connected to a shader of the same shader model.

Channels containing multi-channel layers can be connected to multiple shaders with unique shader models but cannot be connected to multiple similar shaders.

Shader Inputs

A channel containing a multi-channel layer cannot be connected to multiple inputs of the same shader.

Connecting New Channels to Shaders

Multi-channel layers are auto-extended when new channels are connected to the shader.

Article: Read Q100488: Auto Extension and Auto Retraction of materials to learn more about auto-extension behaviour in Mari.

Disconnecting From a Shader

Multi-channel layers are converted within or removed from a channel when it is disconnected from the shader.

Use Cases

Auto-Extension Use Case

Multi-channel layers are auto-extended when new channels are connected to the shader.

On connecting the new channel to the downstream shader, it should recognize the existing materials in layers and auto-extend to the new stream (connected channel) in the Multi-Channel Properties. The new stream, should be Visible or Invisible depending on the option selected in the dialog.

Note: When a new channel is connected to a shader with no upstream multi-channel layers, the dialog will not appear as there is nothing to extend.

Multi-channel layers are converted within or removed from a channel when it is disconnected from the shader.

On disconnecting a channel from the downstream shader, its multi-channel layers should be disconnected, converted into standard layers, or removed depending on the option selected in the dialog.

Retraction Use Cases

Case 1 - Material Layer

Case 2 - Material Layer with Multi-Channel Group

Case 3 - Multi-Channel Group with Standard Paintable Layers

Deleting Shaders

Deleting a shader will invoke retraction of multi-channel layers.

Deleting a shader from the Shaders palette triggers multi-channel retraction, as described above. The Retraction dialog sets the behavior for all Channels orphaned on shader deletion.