This VN has CG or tachie (sprites) whose base shading uses flat tints exclusively, in opposition to gradation.
A flat tint is a coat of water color of one uniform shade when a gradation is a gradual or barely perceptible change from one tone or shade, as of color, to another.
This applies to tachie and event CG, not to backgrounds (usually drawn by a different artist and which can use a totally different art style).
Both techniques can be used in conjunction in a single CG/tachie but they must not use this tag.

Example of flat tints only:
link
link (this one is animated, which explain why this kind of shading is used and why Anime-like shading is an alias of this tag)

Example of gradation (pretty much every VN):
link
link

An example and counter-example (with the skin) using PC98 assets:
Kuro no Danshou
Toumei Ningen

Graphical effects and filters added to the base shading can be applied on a CG and alter the flat tints to the point it adds little gradation to it but it doesn't discard the VN from being tagged as long as the base shading is made of flat tints.
Examples of graphical effects are steam (for onsen events), blurring (for focus), sunset effect, etc. An example of filter would be a "dazzling" filter.

Two examples taken from the same visual novel, Aoiro Note:

Graphical sunset effect.
It adds, from a single point, the sun behind the glass, more light to the base shading with the purpose of making the CG dazzling.

Example of "dazzling" filter.
It makes the whole erotic scene looks like it was taken from a phantasm even though it doesn't have extra light source like it was the case with the previous example.

Now, the base shading without impurities can be seen from the tachie.