. Answer to [Q] from Blender.org
Simple
Creates a new level of subdivision using a simple interpolation by subdividing edges without any smoothing.

Linear
Creates a new level of subdivision using linear interpolation of the current sculpted displacement.

Doesn't really give me a lot of detail, but Blender's explanation of linear vs simple:

https://docs.blender.org/manual/en/latest/modeling/modifiers/generate/multiresolution.html#:~:text=The%20Multiresolution%20modifier%20(often%20shortened,subdivision%20levels%20in%20Sculpt%20Mode.

would be great for us, wainscooters

Nice pun!

[Q] please show us the baking (and dirty vertex)

That sounds good.

The Blender Manual is unfortunately very blurry in its description

just type in ctrl-f and then his name

I just remembered something. We had an ironcast lantern. Cheap metal in olden times. Ironcast were blackened with black polish.

[Q] From the blender manual:

Subdivide
Creates a new level of subdivision using the type specified by Subdivision Type (see below).

Simple
Creates a new level of subdivision using a simple interpolation by subdividing edges without any smoothing.

Linear
Creates a new level of subdivision using linear interpolation of the current sculpted displacement.