Defines of how the material UV layout is cut into logical pieces. When destined to a Material Node, it also defines the resolution per layout block of that material for any extension that is made to that material.
Define that the resolution relationship between a material and any addition to that material, like when adding a hair to a character to make sure that the hair uses up as much texture space as the head alone does.
Allow Mutable to optimize texture space by reorganizing the final UV, like when removing a large chunk of torso mesh because the character now wears a jacket.
Add Block (button): Create a new block in the block editor.
Remove Block (button): Remove the block currently selected at the block editor.
(block size dropdown): Defines the grid partition. There's 1x1, 2x2, 4x4, 8x8, 16x16 and 32x32. A material node uses this to define the resolution of one square of grid. In a material extension, this will define to what size are all the added texture parameters stretched to keep with the ancestor's material resolution-to-block ratio. Here is an example of how the grid size and the resolution interact.
(block editor): Click and drag the yellow squares to move a block around. Click and drag the small purple dot at the lower right corner to change the size of that block. The UV layout that is overlayed in the block editor comes from the skeletal mesh that this layout node is connected to.
Layout: Layout defined in this node. To what skeletal mesh is this ultimately connected, defines what UV will be overlayed in the block editor.