Water retention refers to the tendency of the brush to retain water in the process of use, or to store little water. Brushes with different bristles show different performance in this respect, with badger bristles being those with strong water storage and bristles being those with low water storage. This performance is not strong or weak on the good argument, the degree of individuation is very strong, to be able to match with their own shaving habits as well.
Density, literally, refers to how tightly the bristles are tied, or it can also be interpreted as whether the bristles are dense enough. Generally dense is preferred, but too dense may cause the brush shape to loose. Brushes with low density can be described as floppy, which is a typical poor description. The density mainly depends on how the brush is made, and less on the bristles themselves.
The general evaluation of a shaving brush is a comprehensive evaluation from the above four dimensions.
