You are referring to the quote by JamesK.
I’m not really sure what is meant by “no boundary”.
I thought at first that this referred to the mesh having an outer border, lik a mesh grid versus a mesh UVSphere, but I don’t see how that would enter the realm of the discussion with regards to manifold meshes.
I think the confusion is arising from the terminology and using double negatives in the discussion, ie. not a non manifold mesh
In fact, FlamePixel’s post at Blender.org should have stated the error message as “non manifold mesh” (ie. it is NOT a manifold mesh)therefore can’t decimate it.
So a manifold mesh which can be decimated is like a simple mesh grid where each edge has no more than 2 adjoining faces; whereas a NON manifold mesh is like a ‘Y’ or ‘T’ arrangement of faces like the example image posted by Joeri at Blender.org.
This is in agreement with the defintion posted at the Blender Knowledge base:
I believe the text you referred to in your first post was:
"Title: How do I use the decimator and not get a ‘Not a manifold mesh error’?
FAQ Text: The way the mesh decimator is implemented, it requires each edge to have at most two adjoining faces. If you have more than that, you’ll get this error. "
In other words - for the decimator to function, it must be a manifold mesh in which each edge does NOT have more than 2 adjoining faces, otherwise you have a non-manifold mesh and need to identify and fix the offending part.
Does the deimator have any benefit other than reducing the poly count for exporting a model for use in a game?